HomePhysics ToolsSpring Constant Calculator with Steps | k = F/x, Any Units

Spring Constant Calculator with Steps | k = F/x, Any Units

Spring Constant Calculator

Calculate the spring constant k with steps: from force and extension (Hooke’s law), a hanging mass and its stretch, the oscillation period or frequency, stored energy, springs in series or parallel, coil dimensions, or a force–extension graph of your own lab data. Any units, a units check and a diagram drawn from your numbers.

PhysicsHooke’s LawStep by StepAny Unitsk = F ÷ xFree

⚡ 0. Quick Answer

A spring constant calculator finds the stiffness k of a spring from Hooke’s law: divide the force by the extension, k = F ÷ x. A force of 2.00 N that stretches a spring by 4.00 cm (0.0400 m) gives k = 2.00 ÷ 0.0400 = 50.0 N/m.

k = F ÷ x   [N/m] = [N] ÷ [m]

Rule of thumb: always use the change in length, in metres. For a hanging mass, the force is its weight, so k = m g ÷ x.

Key Takeaways

  • A spring constant calculator uses k = F ÷ x: 2.00 N stretching a spring 4.00 cm gives 50.0 N/m.
  • With a hanging mass, k = m g ÷ x: 0.200 kg stretching a spring 4.90 cm gives 40.0 N/m (g = 9.81 m/s²).
  • Without a ruler, time the bounce: k = 4π²m ÷ T². A 0.500 kg mass with a period of 0.800 s gives 30.8 N/m.
  • Springs in series give 1/k = 1/k₁ + 1/k₂: 200 N/m and 300 N/m end to end give 120 N/m; side by side they give 500 N/m.
  • From a lab graph of force against extension, k is the gradient. The sample data give 40.0 ± 0.3 N/m.

📚 1. What Is a Spring Constant Calculator?

A spring constant calculator works out how stiff a spring is. The spring constant (k), also called the force constant or spring rate, is the force needed to stretch or compress a spring by one metre. It is measured in newtons per metre (N/m). A soft spring in a ballpoint pen might be a few hundred N/m; a car suspension spring is tens of thousands of N/m.

To calculate the spring constant, use Hooke’s law, F = k x, where F is the force on the spring and x is the extension or compression, the change in length from its natural length. Rearranging gives k = F ÷ x. If a mass m hangs at rest on the spring, the force is its weight, so k = m g ÷ x. If you know how fast it bounces instead, use k = 4π²m ÷ T².

A worked one-liner: a force of 2.00 N stretches a spring by 4.00 cm. Converting to metres, x = 0.0400 m, so k = 2.00 N ÷ 0.0400 m = 50.0 N/m. On a graph of force against extension, this is the gradient of the straight line.

Spring constant calculator example: 2.00 N stretches a spring 4.00 cm, so k = 50.0 N/mLeft, an unloaded spring and the same spring stretched by 4.00 cm under 2.00 N. Right, a force against extension graph whose gradient is the spring constant.unloadedx4.00 cmF = 2.00 N01234512Extension x (cm)Force F (N)Δx = 4.00 cmΔF = 2.00 Nk = ΔF ÷ Δx = 50.0 N/m
Hooke’s law: the force on a spring is proportional to its extension. The gradient of the force–extension line is the spring constant k.

Who uses it: students in CBSE and ICSE Class 11, IGCSE and GCSE (the required practical on springs), A level, AP Physics 1, JEE and NEET, and first-year engineering. Engineers use spring rates to design car suspensions, valve springs, mattresses, weighing scales and vibration mounts. Robert Hooke first published the law in 1678 as the Latin anagram ut tensio, sic vis, “as the extension, so the force”.

What you knowSpring constant formulaKey idea
Force and extensionk = F ÷ xHooke’s law, the definition of k
Natural and stretched lengthk = F ÷ (L − L₀)Use the change in length
Hanging mass and stretchk = m g ÷ xAt rest the spring holds the weight
Period of oscillationk = 4π² m ÷ T²No ruler needed; time the bounce
Stored energyk = 2E ÷ x²Elastic potential energy, E = ½ k x²
Springs in series1/k = 1/k₁ + 1/k₂Softer than any one spring
Springs in parallelk = k₁ + k₂Stiffer than any one spring
Coil dimensionsk = G d⁴ ÷ (8 D³ n)Compression and extension springs
Lab graphk = gradient of F against xBest method for real data

🧮 2. Enter Your Values

Pick what you know, choose what to solve for, then type your values in any units.

Solve for:

Formula: k = F ÷ x

Scientific notation works: type 6.2e-3. The extension x is the change in length from the natural length, not the total length. Use the size of a compression with the size of the pushing force.

Result
___
Enter your values and press Calculate to see the answer, its units and every step.

📊 3. Results and Step-by-Step Working

___ Enter your values and press Calculate to see the spring constant with its units and every step, a full results table, a diagram drawn from your numbers and four charts.

🧠 4. Understanding Your Answer, In Detail

___ Press Calculate to fill the first part of this section with your own numbers.

4.1 What k means

The spring constant is the stiffness of a spring. A spring with k = 50.0 N/m needs 50.0 N to stretch it 1 m, 0.500 N to stretch it 1 cm, and 2.00 N to stretch it 4.00 cm. A bigger k means a stiffer spring that stretches less for the same force. The unit N/m is the same as kg/s².

4.2 Extension, not length

The most common mistake is dividing by the total length of the spring. Hooke’s law uses x, the change in length from the natural (unloaded) length. A spring that is 12.0 cm long unloaded and 15.5 cm long with 1.40 N on it has x = 3.5 cm, so k = 1.40 ÷ 0.035 = 40.0 N/m.

4.3 Is it a sensible size?

School lab springs are usually 5 to 100 N/m. A retractable pen spring is around 100 to 500 N/m, a mattress spring a few thousand N/m, and a car suspension spring 20 000 to 100 000 N/m (20 to 100 N/mm). If you get 0.4 N/m or 4000 N/m for a lab spring, check whether the extension was in cm or mm instead of metres.

4.4 Three independent ways to measure k

You can find k statically (hang a mass and measure the stretch), dynamically (time the oscillations and use k = 4π²m ÷ T²) or from a graph of several readings. The two methods should agree within a few percent. If the dynamic value is lower, the spring’s own mass is the usual reason: about one third of it joins the oscillating mass.

4.5 Combining springs

Springs combine the opposite way to resistors. Side by side (parallel), they share the load and k = k₁ + k₂. End to end (series), each carries the full force and stretches by F ÷ kᵢ, so 1/k = 1/k₁ + 1/k₂. Cutting a spring in half doubles its spring constant, because each half is one of two equal springs in series.

4.6 What the charts show

The force–extension chart is the heart of Hooke’s law: a straight line through the origin whose gradient is k. The energy chart is a parabola, because E = ½ k x². For oscillations, a graph of T² against m is a straight line of gradient 4π² ÷ k. The point marked on each chart is your spring.

4.7 Where Hooke’s law breaks down

Hooke’s law holds only up to the limit of proportionality. Past the elastic limit, the spring is permanently deformed and does not return to its original length. Tightly wound extension springs also need a small initial tension before they start to stretch, so their force–extension line does not pass through the origin; use the gradient, not one reading.

4.8 Where this leads next

The spring constant sets the natural frequency of anything that vibrates: a car on its suspension, a building in an earthquake, atoms in a crystal and the quartz in a watch. The same idea, stiffness, appears as Young’s modulus for materials and as the torsion constant for twisting springs.

✍ 5. How to Write the Answer

▶ Press Calculate above to fill all five write-ups with your own numbers: the full GRESS working, every unit, an uncertainty, a mark scheme and a plain-text version.

Example 1, Full exam answer (GRESS working)
___
📌 Conventions
  • Convert every length to metres and every mass to kilograms first.
  • Write Hooke’s law before rearranging it.
  • Round to the smallest number of significant figures in the data.
Example 2, Final answer with full details
___
📌 Conventions
  • The unit of k is N/m, written with a space: 50.0 N/m.
  • Engineers often quote spring rates in N/mm or lbf/in.
  • k is always positive; the minus sign in F = −kx only shows the direction of the force.
Example 3, Lab report results with uncertainty
___
📌 Conventions
  • Name the method: static extension, timed oscillations or graph gradient.
  • For a graph, quote the gradient with its standard error from the Graph tab.
  • The uncertainty here assumes ±1 in the last digit of each value, combined in quadrature.
Example 4, Teacher answer key with mark scheme
___
📌 Conventions
  • Award a mark for converting cm to m.
  • For hanging-mass questions, state which g the key expects.
  • Award a separate mark for the unit N/m.
Example 5, Plain-text working for a forum or chat
___
📌 Conventions
  • Write m/s^2 for m/s², pi for π and e5 for powers of ten.
  • Use * for multiply and / for divide so the line pastes anywhere.

∑ 6. Formulas Used

①
Hooke’s Law: Spring Constant from Force and Extension
F = k x  ⇒  k = F ÷ x
kSpring constant (force constant, spring rate), N/m = kg/s²
FForce stretching or compressing the spring, newtons (N)
xExtension or compression, the change in length, metres (m): x = L − L₀
Example2.00 N ÷ 0.0400 m = 50.0 N/m
②
Spring Constant from Mass and Distance
k x = m g  ⇒  k = m g ÷ x
mHanging mass at rest, kilograms (kg)
gGravitational field strength, m/s² (9.81 on Earth)
Example0.200 kg × 9.81 ÷ 0.0490 m = 40.0 N/m
③
Spring Constant from the Period or Frequency
T = 2π √(m ÷ k)  ⇒  k = 4π² m ÷ T² = 4π² m f²
TPeriod, time for one full oscillation, s; T = t ÷ n for n oscillations in time t
fFrequency, Hz; f = 1 ÷ T
Example4π² × 0.500 ÷ 0.800² = 30.8 N/m
④
Spring Constant from Stored Energy
E = ½ k x²  ⇒  k = 2E ÷ x²
EElastic potential energy stored, joules (J)
Example2 × 0.90 J ÷ (0.060 m)² = 500 N/m
⑤
Equivalent Spring Constant: Series and Parallel
Series: 1/k = 1/k₁ + 1/k₂ + …  ·  Parallel: k = k₁ + k₂ + …
SeriesEnd to end: same force, extensions add; two springs give k₁k₂ ÷ (k₁ + k₂)
ParallelSide by side: same extension, forces add
Example200 × 300 ÷ 500 = 120 N/m in series; 500 N/m in parallel
⑥
Spring Rate from Coil Dimensions
Compression: k = G d⁴ ÷ (8 D³ n)  ·  Torsion: k = E d⁴ ÷ (64 D n)
G, EShear modulus and Young’s modulus of the wire, Pa (steel: 79.3 GPa and 207 GPa)
d, DWire diameter and mean coil diameter, m
nNumber of active coils; torsion rate is in N·m per radian
Example79.3 GPa, d 2.00 mm, D 20.0 mm, n 10: k = 1.98 N/mm
⑦
Graph Gradient and Its Uncertainty
k = SxF ÷ Sxx  ·  SE(k) = √[ Σ(residual²) ÷ (n − 2) ÷ Sxx ]
SxxΣ(x − x̄)², the spread of the extensions
SxFΣ(x − x̄)(F − F̄)
Units1 N/m = 1 kg/s²; 1 N/mm = 1000 N/m; 1 lbf/in = 175.127 N/m

🔀 7. Spring Constant Calculation Flowchart, Step by Step

This flowchart shows how to calculate the spring constant step by step, from a word problem to a checked answer. Every box uses the same worked example as the Excel, R and Python sections: a force of 2.00 N stretches a spring by 4.00 cm. Find the spring constant.

Start or result Data step Calculation step Yes or no check
Start 2.00 N stretches a spring 4.00 cm Find k.
1
List the givens F = 2.00 N, x = 4.00 cm
CheckIs x the change in length, not the total length?
YesUse it as it is.
NoFind x = L − L₀ first.
2
Convert to SI units x = 4.00 cm ÷ 100 = 0.0400 m
CheckDo you know the force directly?
YesUse F as given.
NoFor a hanging mass, F = m g.
3
Write Hooke’s law and rearrange F = k x  ⇒  k = F ÷ x
4
Substitute with units k = 2.00 N ÷ 0.0400 m = 50.0 N/m
CheckDoes k x give back F?
Yes50.0 × 0.0400 = 2.00 N.
No, stopRecheck the unit conversion.
Result k = 50.0 N/m Units check ✓ Verified The same spring would stretch 10.0 cm under 5.00 N.

Check any box yourself: the Excel sheet in Section 9, the R script in Section 10 and the Python script in Section 11 all return 50.0 N/m for this spring. Section 14 shows the method written out by hand.

📝 8. How to Use This Calculator

  1. Choose what you know. Pick force and extension, a hanging mass, the oscillation period, stored energy, springs in series or parallel, coil dimensions, or the Graph tab for lab data.
  2. Choose what to solve for. Click a Solve for chip, for example the spring constant, the force, the extension, the mass or the period. That field turns into a dashed answer box.
  3. Type the known values. Plain numbers, decimals and scientific notation all work: 2.00, 0.0400 or 4e-2.
  4. Pick the unit beside each value. Grams, pounds, mm, cm, inches, lbf, N/mm, lbf/in, GPa and psi are converted for you.
  5. Use the change in length. Enter the extension or compression, or use the two-lengths option and let the calculator subtract.
  6. Set g for a hanging mass. Use 9.81 for IGCSE, A level and most US courses, 9.8 for CBSE and AP, or 10 for quick estimates.
  7. Press Calculate. Nothing is worked out until you do, and changing any input clears the old answer so you never read stale numbers.
  8. Read the answer box and the badges. They show k with its units, other units, Units check ✓ and Verified.
  9. Follow the working, the diagram and the charts. The GRESS table, numbered steps, results table, diagram and four charts explain the answer.
  10. Copy or download the working. Section 5 gives five ready-to-paste write-ups, and Download Doc or Download PDF saves the whole solution.

📈 9. How to Calculate the Spring Constant in Excel

Excel has no spring constant function, and it does not need one. k = F ÷ x is one division, so a sheet with one spring per row handles a whole class set. The two habits that matter are converting every extension to metres in its own column, and using SLOPE on a set of readings rather than a single division.

The whole thing in one line: force in B2 (N) and extension in C2 (cm), then type =B2/(C2/100). For 2.00 N and 4.00 cm, Excel returns 50, in N/m.

9.1 The functions you need

You wantExcel formulaNote
Extension in metres=C2/100From cm; use /1000 for mm
Spring constant from force=B2/D2k = F ÷ x with x in metres
Spring constant from a hanging mass=B2/1000*$H$1/D2Mass in grams in B2, g in H1
Spring constant from the period=4*PI()^2*B2/C2^2Mass in kg in B2, period in s in C2
Spring constant from energy=2*B2/D2^2Energy in J in B2
Two springs in series=1/(1/B2+1/C2)Add a +1/D2 for a third spring
Springs in parallel=SUM(B2:D2)k = k₁ + k₂ + k₃
Gradient of a lab graph=SLOPE(C2:C7,D2:D7)Force column first, then extension
Standard error of the gradient=INDEX(LINEST(C2:C7,D2:D7,TRUE,TRUE),2,1)The ± on k
Coil spring rate=B2*1E9*(C2/1000)^4/(8*(D2/1000)^3*E2)G in GPa, d and D in mm, n coils

9.2 Step by step

1 Set up the columns

One spring per row, units in every header. Type the extension as you measured it, in centimetres.

Excel sheet springs.xlsx: cell C2 contains 4Xsprings.xlsx - ExcelC2fx4ABCDEFGH1SpringF (N)x (cm)x (m)k (N/m)g (m/s²)9.812Lab spring2.004.003Pen spring3.00.804Car spring40006.0

Three springs in rows 2 to 4. Column D will hold the extension in metres.

2 Convert the extension to metres

=C2/100

Hooke’s law needs SI units. Fill the formula down to row 4.

Excel sheet springs.xlsx: cell D2 contains =C2/100Xsprings.xlsx - ExcelD2fx=C2/100ABCDEFGH1SpringF (N)x (cm)x (m)k (N/m)g (m/s²)9.812Lab spring2.004.000.043Pen spring3.00.800.0084Car spring40006.00.06

4.00 cm becomes 0.04 m.

3 Divide force by extension

=B2/D2

This is k = F ÷ x. Fill it down.

Excel sheet springs.xlsx: cell E2 contains =B2/D2Xsprings.xlsx - ExcelE2fx=B2/D2ABCDEFGH1SpringF (N)x (cm)x (m)k (N/m)g (m/s²)9.812Lab spring2.004.000.04503Pen spring3.00.800.0083754Car spring40006.00.0666666.66667

The lab spring gives 50 N/m; format the cell to one decimal place to show 50.0.

4 Find k from a graph of readings

=SLOPE(C2:C7,D2:D7)

On a second sheet, put the masses in B (g), the force in C (=B2/1000*$H$1) and the extension in metres in D. SLOPE gives the gradient.

Excel sheet springs.xlsx: cell F2 contains =SLOPE(C2:C7,D2:D7)Xsprings.xlsx - ExcelF2fx=SLOPE(C2:C7,D2:D7)ABCDEFGH1x (cm)m (g)F (N)x (m)k (N/m)g (m/s²)9.812000040.007193132.41000.9810.024452001.9620.0557.33002.9430.07369.94003.9240.099712.25004.9050.122

Six readings give k = 40.0 N/m, with a standard error of 0.32 N/m from LINEST.

9.3 The awkward cases

SLOPE takes y first, then x. =SLOPE(D2:D7,C2:C7) gives extension per newton, 0.025 m/N, the reciprocal of k. Put the force column first so the answer is in N/m.
  • Initial tension. A closely wound extension spring needs some force before it stretches at all. Its line does not pass through zero, so use SLOPE, not a single F ÷ x.
  • Two lengths. If you recorded lengths, not extensions, make a column =C2-$C$2 to subtract the unloaded length from every row.
  • Units of spring rate. Divide N/m by 1000 for N/mm, or use =CONVERT(E2,"N","lbf")*0.0254 for lbf/in.
  • Precision: Excel keeps about 15 significant digits. Round only in the final display with =ROUND(E2,1) or the number format.

9.4 Why Excel might disagree with R, Python or this page

Value of gk for 0.200 kg stretching 4.90 cmWho uses it
9.80665 m/s²40.027 N/m → 40.0 N/mStandard gravity; scipy.constants.g in Python
9.81 m/s²40.041 N/m → 40.0 N/mIGCSE, A level, most US textbooks, this page
9.8 m/s²40.0 N/mCBSE, NCERT, AP Physics
10 m/s²40.8 N/mQuick estimates and some exam boards

For force-and-extension problems g does not appear, so every tool gives exactly the same k. When a mass is involved, a different last digit is usually a different g. A graph gradient also depends on whether you force the line through zero: LINEST(C2:C7,D2:D7,FALSE) gives 40.0 N/m for the sample data, and the free line gives 40.0 N/m too, because the intercept is tiny.

9.5 Charting it

  1. Put the extension (m) in one column and the force (N) in the next.
  2. Insert → Charts → Scatter. Do not use the Line chart type, which spaces x values evenly.
  3. Click a point → Add Trendline → Linear → tick Display Equation on chart and Display R-squared.
  4. The number in front of x in the equation is k. An intercept far from zero points to a zero error or initial tension.
  5. Label the axes with units: Extension x (m) and Force F (N).

9.6 Errors you will hit

Excel showsWhyFix
#VALUE!A unit typed in a cell, such as "4 cm"Keep units in the header; cells hold numbers only
#DIV/0!Extension is zero or blankFill the cell, or wrap with =IFERROR(...,"")
k is 100 times too smallExtension left in cm, divided by 100 twice, or k divided by 100Convert once, in its own column
k is 100 times too bigExtension used in cm instead of mDivide the cm value by 100
SLOPE answer is about 0.025x and y swappedForce range first: =SLOPE(force, extension)
Every row gives the same answerRelative reference to g changed when you filled downUse $H$1, not H1

📈 10. How to Calculate the Spring Constant in R

R works out the spring constant in one line, k <- F / x. The script below does more: it finds k from a hanging mass, the period, stored energy, series and parallel springs and coil dimensions, fits a straight line to lab readings with lm() for the gradient and its standard error, runs a substitution check, and saves one 300 dpi figure of force against extension. It uses base R only.

Copy the whole block. It runs top to bottom in RStudio, Posit Cloud or Rscript sk.R. Change only the lines under "Your values" and the two data vectors to solve your own problem.

10.1 The complete script

R, base only, saves spring_constant_plot.png

# ============================================================
# Spring constant calculator in R, with one 300 dpi figure
# StatsUnlock - https://statsunlock.com/spring-constant-calculator/
# Base R only. No packages to install.
# ============================================================

# ---- 1. Your values (SI units) -----------------------------
F <- 2.00          # force on the spring in N
x <- 4.00 / 100    # extension in m (4.00 cm)
g <- 9.81          # m/s^2; use 9.80665, 9.8 or 10 to match your book

# ---- 2. Hooke's law: k = F / x -----------------------------
k <- F / x                                   # N/m

# ---- 3. Other ways to find k -------------------------------
k_mass   <- 0.200 * g / (4.90 / 100)         # hanging mass 0.200 kg, stretch 4.90 cm
k_period <- 4 * pi^2 * 0.500 / 0.800^2       # 0.500 kg bouncing with T = 0.800 s
k_energy <- 2 * 0.90 / (6.0 / 100)^2         # 0.90 J stored at 6.0 cm
k_series <- 1 / (1 / 200 + 1 / 300)          # 200 N/m and 300 N/m end to end
k_par    <- 200 + 300                        # the same springs side by side
k_coil   <- 79.3e9 * (2.00e-3)^4 / (8 * (20.0e-3)^3 * 10)   # G d^4 / (8 D^3 n)

# ---- 4. From a graph: least-squares gradient ---------------
ext  <- c(0, 2.4, 5.0, 7.3, 9.9, 12.2) / 100        # extension, m
load <- c(0, 100, 200, 300, 400, 500) / 1000 * g    # weight of the masses, N
fit  <- lm(load ~ ext)
k_graph <- unname(coef(fit)[2])
se      <- summary(fit)$coefficients[2, 2]

# ---- 5. Print the answers with units -----------------------
cat(sprintf("Hooke's law k     = %.1f N/m\n", k))
cat(sprintf("From hanging mass = %.1f N/m\n", k_mass))
cat(sprintf("From the period   = %.3g N/m\n", k_period))
cat(sprintf("From the energy   = %.0f N/m\n", k_energy))
cat(sprintf("Series / parallel = %.3g / %.3g N/m\n", k_series, k_par))
cat(sprintf("Coil spring rate  = %.3g N/mm\n", k_coil / 1000))
cat(sprintf("Graph gradient    = %.1f +/- %.2f N/m\n", k_graph, se))
stopifnot(abs(k * x - F) < 1e-12)             # substitution check
cat("Units: N / m = kg/s^2 = N/m\n")

# ---- 6. One figure: force against extension ----------------
png("spring_constant_plot.png", width = 7, height = 4.5, units = "in", res = 300)
plot(ext * 100, load, pch = 19, col = "#2563eb", cex = 1.3,
     xlab = "Extension x (cm)", ylab = "Force F (N)",
     main = sprintf("Gradient k = %.1f N/m", k_graph))
abline(coef(fit)[1], coef(fit)[2] / 100, lwd = 3, col = "#9333ea")
legend("topleft", c("Measured", "Best-fit line"), col = c("#2563eb", "#9333ea"),
       pch = c(19, NA), lwd = c(NA, 3), bty = "n")
invisible(dev.off())

Output when run: Hooke's law k = 50.0 N/m, From hanging mass = 40.0 N/m, From the period = 30.8 N/m, From the energy = 500 N/m, Series / parallel = 120 / 500 N/m, Coil spring rate = 1.98 N/mm, Graph gradient = 40.0 +/- 0.32 N/m. These match Sections 3, 7, 9 and 11.

10.2 What each part does

LineWhat it does
x <- 4.00 / 100Converts 4.00 cm to metres before anything else.
k <- F / xHooke’s law rearranged: spring constant in N/m.
4 * pi^2 * 0.500 / 0.800^2k from a 0.500 kg mass bouncing with a period of 0.800 s.
1 / (1 / 200 + 1 / 300)Two springs in series: the reciprocals add.
79.3e9 * (2.00e-3)^4 / (8 * (20.0e-3)^3 * 10)Rate of a steel compression spring from its dimensions, G d⁴ ÷ (8 D³ n).
lm(load ~ ext)Fits force = intercept + k × extension; the gradient is k.
summary(fit)$coefficients[2, 2]Standard error of the gradient, the ± on k.
png(..., res = 300)Saves a print-quality figure; dev.off() closes the file.

R traps: lm(ext ~ load) swaps the axes and returns 1/k; -2^2 is −4 in R; F is also R’s shortcut for FALSE, so the script overwrites it on purpose and you should write FALSE in full afterwards.

10.3 What the figure shows

The blue points are the six readings of force against extension, and the purple line is the least-squares fit. The points lie almost exactly on a straight line through the origin, which is Hooke’s law. The gradient, 40.0 N/m, is the spring constant. If the top points bent away from the line, the spring would be past its limit of proportionality and those points should be left out of the fit.

10.4 Common problems and fixes

ProblemFix
k is 100 times too bigThe extension was in centimetres. Divide by 100.
Gradient is about 0.025The formula is backwards. Use lm(force ~ extension).
The PNG is blankdev.off() did not run; run the last line again.
Later code using F as FALSE breaksRename the force to Fn, or write FALSE in full.

10.5 Useful one-liners

# k for several springs at once (force in N, extension in cm)
c(2.00, 3.0, 5.0) / (c(4.00, 0.80, 2.5) / 100)
# k from 20 oscillations of 0.250 kg in 15.8 s
4 * pi^2 * 0.250 / (15.8 / 20)^2
# line forced through the origin
coef(lm(load ~ 0 + ext))
# 95% confidence interval for k
confint(fit)[2, ]

📈 11. How to Calculate the Spring Constant in Python

Python does the same job with NumPy and Matplotlib. The script finds k from force and extension, a hanging mass, the period, stored energy, series and parallel springs and coil dimensions, fits the lab readings with np.polyfit, prints each answer with its unit, and saves one 300 dpi figure of force against extension.

numpyArrays, pi and the straight-line fit
matplotlibThe 300 dpi force–extension plot

Install once with pip install numpy matplotlib. Google Colab already has both.

11.1 The complete script

Python 3 with NumPy, saves spring_constant_plot.png

# ============================================================
# Spring constant calculator in Python, with one 300 dpi figure
# StatsUnlock - https://statsunlock.com/spring-constant-calculator/
# pip install numpy matplotlib
# ============================================================
import numpy as np
import matplotlib.pyplot as plt

# ---- 1. Your values (SI units) -----------------------------
F = 2.00            # force on the spring, N
x = 4.00 / 100      # extension, m (4.00 cm)
g = 9.81            # m/s^2 (scipy.constants.g would give 9.80665)

# ---- 2. Hooke's law: k = F / x -----------------------------
k = F / x           # N/m

# ---- 3. Other ways to find k -------------------------------
k_mass = 0.200 * g / (4.90 / 100)
k_period = 4 * np.pi**2 * 0.500 / 0.800**2
k_energy = 2 * 0.90 / (6.0 / 100)**2
k_series = 1 / (1 / 200 + 1 / 300)
k_par = 200 + 300
k_coil = 79.3e9 * (2.00e-3)**4 / (8 * (20.0e-3)**3 * 10)

# ---- 4. From a graph: least-squares gradient ---------------
ext = np.array([0, 2.4, 5.0, 7.3, 9.9, 12.2]) / 100
load = np.array([0, 100, 200, 300, 400, 500]) / 1000 * g
k_graph, c = np.polyfit(ext, load, 1)                 # gradient and intercept
res = load - (k_graph * ext + c)
se = np.sqrt((res**2).sum() / (len(ext) - 2) / ((ext - ext.mean())**2).sum())   # standard error of the gradient

# ---- 5. Print the answers with units -----------------------
print(f"Hooke's law k     = {k:.1f} N/m")
print(f"From hanging mass = {k_mass:.1f} N/m")
print(f"From the period   = {k_period:.3g} N/m")
print(f"From the energy   = {k_energy:.0f} N/m")
print(f"Series / parallel = {k_series:.3g} / {k_par:.3g} N/m")
print(f"Coil spring rate  = {k_coil / 1000:.3g} N/mm")
print(f"Graph gradient    = {k_graph:.1f} +/- {se:.2f} N/m")
assert abs(k * x - F) < 1e-12          # substitution check

# ---- 6. One figure: force against extension ----------------
xs = np.linspace(0, ext.max(), 50)
fig, ax = plt.subplots(figsize=(7, 4.5))
ax.plot(ext * 100, load, "o", color="#2563eb", ms=7, label="Measured")
ax.plot(xs * 100, k_graph * xs + c, lw=3, color="#9333ea", label="Best-fit line")
ax.set_xlabel("Extension x (cm)")
ax.set_ylabel("Force F (N)")
ax.set_title(f"Gradient k = {k_graph:.1f} N/m")
ax.legend(frameon=False)
fig.savefig("spring_constant_plot.png", dpi=300, bbox_inches="tight")

Output when run: Hooke's law k = 50.0 N/m, From hanging mass = 40.0 N/m, From the period = 30.8 N/m, From the energy = 500 N/m, Series / parallel = 120 / 500 N/m, Coil spring rate = 1.98 N/mm, Graph gradient = 40.0 +/- 0.32 N/m.

11.2 What each part does

LineWhat it does
k = F / xHooke’s law rearranged, N/m.
4 * np.pi**2 * 0.500 / 0.800**2k from the period; powers use **, not ^.
np.polyfit(ext, load, 1)Straight-line fit; the first number returned is the gradient k.
res = load - (k_graph * ext + c)Residuals: how far each point lies from the line.
np.sqrt(... / (len(ext) - 2) / ...)Standard error of the gradient, the same value as R’s lm() and Excel’s LINEST.
f"{k:.1f}"Formats to one decimal place, keeping the trailing zero in 50.0.
fig.savefig(..., dpi=300)Saves a print-quality PNG.

Python traps: ^ is not a power; f"{50.0:.3g}" prints 50, dropping the significant zero; np.polyfit(load, ext, 1) swaps the axes and returns 1/k; scipy.constants.g is 9.80665.

11.3 What the figure shows

The same force–extension graph as in R: six readings on a straight line through the origin, with a gradient of 40.0 N/m. Change the two arrays at the top of the script to your own readings and the title updates with your spring constant.

11.4 Common problems and fixes

ProblemFix
ModuleNotFoundError: numpyRun pip install numpy matplotlib, or use Google Colab.
k is 100 times too bigDivide extensions in cm by 100 before fitting.
No PNG appearsLook in the folder you ran the script from, or print os.getcwd().
Last digit differs from your textbookSet g to the value your course uses.

11.5 Useful one-liners

# k for several springs at once (force in N, extension in cm)
import numpy as np; np.array([2.00, 3.0, 5.0]) / (np.array([4.00, 0.80, 2.5]) / 100)
# k from 20 oscillations of 0.250 kg in 15.8 s
4 * np.pi**2 * 0.250 / (15.8 / 20)**2
# line forced through the origin
(ext @ load) / (ext @ ext)
# three springs in series
1 / sum(1 / np.array([200, 300, 600]))

Run it in a terminal with python sk.py, in a Jupyter notebook cell, or in Google Colab without installing anything.

📋 12. Reference Tables

12.1 Spring constant formulas at a glance

You knowSpring constantUnit check
Force F and extension xF ÷ xN ÷ m
Force and two lengthsF ÷ (L − L₀)N ÷ m
Hanging mass m and stretch xm g ÷ xkg × m/s² ÷ m
Mass and period T4π² m ÷ T²kg ÷ s²
Mass and frequency f4π² m f²kg × Hz²
n oscillations in time t4π² m n² ÷ t²kg ÷ s²
Energy E and extension x2E ÷ x²J ÷ m²
Springs in series1 ÷ (1/k₁ + 1/k₂ + …)N/m
Springs in parallelk₁ + k₂ + …N/m
A spring cut into n equal pieceseach piece n kN/m
Coil dimensionsG d⁴ ÷ (8 D³ n)Pa × m
Torsion springE d⁴ ÷ (64 D n)N·m per radian

12.2 Typical spring constants

SpringTypical kNote
Slinky toyabout 1 N/mVery soft
School lab spring5 to 100 N/mStretches several cm under 100 g masses
Ballpoint pen spring100 to 500 N/mSmall and stiff
Trampoline springabout 5 kN/mEach of many in parallel
Car suspension spring20 to 100 kN/mOften quoted as 20 to 100 N/mm
Engine valve spring30 to 100 kN/mVery stiff, short travel

Typical published ranges; real springs vary widely. Measure your own spring when accuracy matters.

12.3 Spring wire materials

MaterialShear modulus GYoung’s modulus E
Music wire (high-carbon steel)79.3 GPa (11.5 Mpsi)207 GPa (30 Mpsi)
Stainless steel 30269.0 GPa (10.0 Mpsi)193 GPa (28 Mpsi)
Chrome silicon77.2 GPa (11.2 Mpsi)203 GPa (29.5 Mpsi)
Beryllium copper48.3 GPa (7.0 Mpsi)128 GPa (18.5 Mpsi)
Phosphor bronze41.4 GPa (6.0 Mpsi)103 GPa (15 Mpsi)

Standard spring-design values for wire about 1 to 3 mm in diameter. G is used for compression and extension springs; E is used for torsion springs.

12.4 Spring rate unit conversions

UnitIn N/mWhere you meet it
1 N/cm100 N/mSchool practicals
1 N/mm1000 N/mSpring catalogues, car suspension
1 kN/m1000 N/mEngineering
1 lbf/in175.127 N/mUS spring catalogues, coilovers
1 kgf/mm9806.65 N/mJapanese and European suspension specs

12.5 Constants

ConstantValueStatus
Standard acceleration of gravity g₀9.806 65 m/s²exact (defined)
Typical surface g on Earth9.78 (equator) to 9.83 (poles) m/s²measured
4π²39.478 4mathematical constant
1 lbf4.448 222 Nexact by definition

Source: NIST, CODATA 2022 recommended values (Mohr et al., 2025, reference 1).

📈 13. Example Results

1
NON-SI INPUTS

A 450 lbf/in coilover spring

Pounds per inch, N/mm and N/m.

A car coilover spring is rated at 450 lbf/in. Convert the spring rate to N/mm and find the force needed to compress it by 2.0 in.

k = 450 lbf/in= 78.8 N/mmx = 2.0 inF = 900 lbf ≈ 4.0 kN
QuantityValueNote
1 lbf/in in N/m175.127 N/m4.448 N ÷ 0.0254 m
k in N/m78,807 N/m450 × 175.127
F = k x900 lbf = 4,003 N450 × 2.0 in
Coilover: 450 lbf/in is 78.8 N/mm, inside the typical car rangeSpring rate (N/mm)78.8 N/mmTypical car spring, low (N/mm)20.0 N/mmTypical car spring, high (N/mm)100 N/mm
Spring catalogues mix lbf/in, N/mm and kgf/mm. Converting to N/m first avoids most mistakes.

What it means: 450 lbf/in × 175.127 = 78,807 N/m, which is 78.8 N/mm. Compressing it 2.0 in (50.8 mm) takes 450 × 2.0 = 900 lbf, about 4.0 kN.

How to write it: "k = 450 lbf/in × 175.127 (N/m)/(lbf/in) = 7.88 × 10⁴ N/m = 78.8 N/mm; F = kx = 900 lbf ≈ 4.0 kN."

2
EVERYDAY

The spring inside a 10 N newton meter

Full-scale force and full-scale stretch.

A newton meter reads 10.0 N at full scale, when its spring has stretched 8.00 cm. Find the spring constant and the stretch at 3.5 N.

F = 10.0 Nx = 8.00 cmk = 125 N/mx at 3.5 N = 2.8 cm
QuantityValueNote
k = F ÷ x10.0 ÷ 0.0800 = 125 N/m3 s.f.
x = F ÷ k at 3.5 N0.028 m = 2.8 cmLinear scale
Scale spacing0.800 cm per newton1 ÷ k
Newton meter spring: 10.0 N stretches it 8.00 cm, so k is 125 N/munloadedx = 8.00 cmF = 10.0 Nk = 10.0 N ÷ 0.0800 m = 125 N/m
Because the stretch is proportional to the force, the scale of a newton meter is evenly spaced.

What it means: k = 10.0 N ÷ 0.0800 m = 125 N/m. Every newton stretches the spring 0.800 cm, so at 3.5 N it stretches 2.8 cm. Overloading past 10 N could take it beyond the elastic limit.

How to write it: "k = F/x = 10.0 N ÷ 0.0800 m = 125 N/m."

3
EXAM: SOLVE FOR m

How heavy is the mystery mass?

Using a known spring as a scale.

A spring with k = 25 N/m stretches 6.0 cm when an unknown mass hangs from it at rest. Take g = 9.8 m/s² (CBSE style). Find the mass.

k = 25 N/mx = 6.0 cmg = 9.8 m/s²m = 153 g
QuantityValueNote
Spring force k x1.5 N25 × 0.060
m = k x ÷ g0.1531 kg → 0.15 kg2 s.f.
In grams150 g
Unknown mass: a 25 N/m spring stretched 6.0 cm holds 0.15 kgunloadedx = 6.0 cm? kgmg = 1.5 Nm = 25 × 0.060 ÷ 9.8 = 0.15 kg
At rest the spring force k x balances the weight m g.

What it means: k x = 25 × 0.060 = 1.5 N, which equals m g, so m = 1.5 ÷ 9.8 = 0.153 kg, about 150 g.

How to write it: "m = kx/g = (25 N/m)(0.060 m) ÷ 9.8 m/s² = 0.15 kg."

4
ENGINEERING

Ride frequency of a car

Four springs in parallel and the bounce rate.

A 1200 kg car sits on four suspension springs, each 30 kN/m. Find the total spring constant and the natural bounce frequency.

4 × 30 kN/m in parallelk = 120 kN/mm = 1200 kgf = 1.59 Hz
QuantityValueNote
k = k₁ + k₂ + k₃ + k₄120 000 N/mParallel springs add
ω = √(k ÷ m)10.0 rad/s√(120 000 ÷ 1200)
f = ω ÷ 2π1.592 Hz → 1.6 HzComfortable ride: 1 to 1.5 Hz
Car: four 30 kN/m springs in parallel make 120 kN/mOne spring (kN/m)30.0 kN/mFour in parallel (kN/m)120 kN/m
The four corner springs share the car’s weight, so their spring constants add.

What it means: k = 4 × 30 kN/m = 120 kN/m. ω = √(120 000 ÷ 1200) = 10.0 rad/s, so f = 10.0 ÷ 2π = 1.59 Hz. Softer springs would give a gentler ride but more body roll.

How to write it: "k = 4 × 30 kN/m = 1.2 × 10⁵ N/m; f = (1/2π)√(k/m) = 1.6 Hz."

5
TINY MAGNITUDE

An atomic force microscope cantilever

Nanometres and nanonewtons.

The cantilever of an atomic force microscope has a spring constant of 0.10 N/m. It bends by 20 nm over a sample. Find the force.

k = 0.10 N/mx = 20 nm = 2.0 × 10⁻⁸ mF = 2.0 nN
QuantityValueNote
Convert2.0 × 10⁻⁸ m20 × 10⁻⁹
F = k x2.0 × 10⁻⁹ N2 s.f.
In nN2.0 nN
AFM: 2.0 nN, a fifth of the weight of a 1 microgram dust grainCantilever force (nN)2.00 nNWeight of a 1 μg dust grain (nN)9.81 nN
Hooke’s law works from car springs down to atomic scales; only the units change.

What it means: F = 0.10 N/m × 2.0 × 10⁻⁸ m = 2.0 × 10⁻⁹ N. Measuring the bend with a laser lets the microscope feel forces between single atoms.

How to write it: "F = kx = 0.10 N/m × 2.0 × 10⁻⁸ m = 2.0 × 10⁻⁹ N."

6
GRAPH METHOD

Spotting the limit of proportionality

Leave the curved point out of the fit.

A spring stretches 2.0, 4.1, 6.0, 8.1, 10.0 and 13.5 cm under loads of 1 to 6 N. Find k from the graph.

6 readingslast point off the linek = 50.0 N/mfrom the first 5
QuantityValueNote
Gradient, first 5 points49.99 N/m → 50 N/mStraight part only
Expected x at 6 N12.0 cmMeasured 13.5 cm
VerdictPast the limit of proportionalityDo not load further
Force against extension: five points on a line with gradient 50 N/m, the sixth bends away024681012140123456Extension (cm)Force (N)k = 50.0 N/m (first 5 points)
The orange point lies off the line: the spring has passed its limit of proportionality.

What it means: The first five points fit a line of gradient 50.0 N/m. At 6 N Hooke’s law predicts 12.0 cm, but the spring stretched 13.5 cm. It has gone past its limit of proportionality and may not return to its original length.

How to write it: "k = gradient of F against x (first five points) = 50 N/m; the 6 N reading is beyond the limit of proportionality."

7
LIMIT CASE

Cutting a spring in half

Why a shorter spring is stiffer.

An 80 N/m spring is cut into two equal halves. What is the spring constant of each half?

k = 80 N/mtwo equal halvesk_half = 160 N/m
QuantityValueNote
Original = two halves in series1/80 = 1/k + 1/k
Solvek = 2 × 80 = 160 N/m
One third of the coils240 N/mn pieces: n k
Cut spring: an 80 N/m spring is two 160 N/m halves in serieswhole: 80 N/mhalf: 160 N/mhalf: 160 N/m1/80 = 1/160 + 1/160
Each half carries the same force but has only half the coils to share the stretch.

What it means: The whole spring is two identical halves in series, so 1/80 = 2/k and each half has k = 160 N/m. Under the same 8 N, the whole spring stretches 10 cm but each half alone stretches only 5 cm.

How to write it: "1/k_whole = 1/k_half + 1/k_half ⇒ k_half = 2 × 80 N/m = 160 N/m."

8
TRAP: TOTAL LENGTH

Length is not extension

The mistake that gives k five times too small.

A spring is 10.0 cm long unloaded and 12.5 cm long with 0.50 N hanging on it. One student gets k = 4.0 N/m. Find the correct value.

L₀ = 10.0 cmL = 12.5 cmx = 2.5 cmk = 20 N/mwrong: 4.0 N/m
QuantityValueNote
x = L − L₀2.5 cm = 0.025 mChange in length
k = F ÷ x0.50 ÷ 0.025 = 20 N/mCorrect
F ÷ L (wrong)0.50 ÷ 0.125 = 4.0 N/mUsed total length
Length trap: 0.50 N stretches a 10.0 cm spring to 12.5 cm, so x is 2.5 cm and k is 20 N/mL₀ = 10.0 cmx = 2.5 cmF = 0.50 Nx = 12.5 − 10.0 = 2.5 cm, k = 20 N/m
Hooke’s law uses the stretch, the dashed gap, not the full length of the spring.

What it means: The extension is 12.5 − 10.0 = 2.5 cm = 0.025 m, so k = 0.50 ÷ 0.025 = 20 N/m. Dividing by the whole length, 0.125 m, gives 4.0 N/m, five times too small.

How to write it: "x = L − L₀ = 0.025 m; k = F/x = 0.50 N ÷ 0.025 m = 20 N/m."

✏️ 14. How to Calculate the Spring Constant by Hand, Step by Step

This is the written method examiners expect, worked on the same example as Sections 7, 9, 10 and 11: a force of 2.00 N stretches a spring by 4.00 cm. Find the spring constant, then the force needed to stretch it by 7.00 cm.

14.1 The written method (GRESS)

  1. Given. F = 2.00 N, x = 4.00 cm = 0.0400 m.
  2. Required. Spring constant k (N/m), then the force for x = 7.00 cm.
  3. Equation. Hooke’s law, F = k x, so k = F ÷ x.
  4. Substitute, with units. k = 2.00 N ÷ 0.0400 m. Then F = k × 0.0700 m.
  5. Solve. k = 50.0 N/m. F = 50.0 × 0.0700 = 3.50 N.
  6. Check the units and the size. N ÷ m = N/m. A longer stretch needs a bigger force, in proportion: 7.00 ÷ 4.00 × 2.00 N = 3.50 N.
  7. Write the answer with units. k = 50.0 N/m; F = 3.50 N.

14.2 On a scientific calculator

TaskKeysWhat you see
Force and extension2 ÷ 0.04 =50
Hanging mass and stretch0.2 × 9.81 ÷ 0.049 =40.0408...
From the period4 × π x² × 0.5 ÷ 0.8 x² =30.842...
Two springs in series( 200 x⁻¹ + 300 x⁻¹ ) x⁻¹ =120
Casio fx-991EX (ClassWiz)Statistics mode, A+BX, enter extension and force, then read B40.007...

The statistics mode on most scientific calculators does linear regression: enter extension as x and force as y, and the coefficient B is the spring constant. Make sure the extensions are in metres first.

14.3 Measure it yourself: a school lab practical

Hooke’s law practical: finding k from a force–extension graph

  1. Clamp a spring to a stand and hang a metre rule beside it. Fix a pointer to the bottom of the spring and read its position with no load.
  2. Hang a 100 g mass hanger on the spring. Wait for it to settle and read the pointer again. The extension is the new reading minus the unloaded reading.
  3. Add 100 g at a time up to 500 g, recording the extension each time. Then remove the masses one by one and check that the spring returns to its original length.
  4. Calculate the force on the spring for each load, F = m g, with m in kilograms.
  5. Plot force (y-axis) against extension (x-axis) in metres. Draw the best-fit straight line through the origin. Its gradient is k.
  6. Check k a second way: hang 300 g, pull it down 2 cm, let go and time 10 oscillations. Use k = 4π²m ÷ T² with T = time ÷ 10.

Main errors: parallax when reading the rule, a pointer that moves, overloading the spring past its elastic limit, and the spring still bouncing when you read it. Read at eye level, wait until the mass is still, and stop adding mass if the readings stop rising evenly.

Mass (g)Force F = mg (N)Extension (cm)Extension (m)F ÷ x (N/m)
1000.9812.40.02440.9
2001.9625.00.05039.2
3002.9437.30.07340.3
4003.9249.90.09939.6
5004.90512.20.12240.2

Sample class data with g = 9.81 m/s². The single F ÷ x values scatter from 39.2 to 40.9 N/m; the gradient of the graph, 40.0 ± 0.3 N/m, is more reliable. These are the data preloaded in the Graph tab.

14.4 How to calculate the uncertainty of the spring constant

For a single reading, add the percentage uncertainties: if F = 2.00 ± 0.02 N (1%) and x = 4.00 ± 0.05 cm (1.25%), then k = 50.0 N/m ± 2.25%, so 50.0 ± 1.1 N/m. For a graph, draw the steepest and shallowest lines that still pass through every error bar; half the difference of their gradients is the uncertainty in k. The Graph tab gives the statistical version, the standard error of the gradient, directly.

14.5 Practice problems with answers

1. A force of 6.0 N stretches a spring by 3.0 cm. Find the spring constant.

x = 0.030 m. k = 6.0 ÷ 0.030 = 200 N/m.

2. A spring is 15.0 cm long with no load and 18.6 cm long when a 4.68 N weight hangs from it. Find k.

Use the extension, not the length: x = 18.6 − 15.0 = 3.6 cm = 0.036 m. k = 4.68 ÷ 0.036 = 130 N/m (2 s.f., because the extension has 2 s.f.).

3. A 500 g mass stretches a spring by 8.0 cm. Find k (g = 9.8 m/s²).

F = 0.500 × 9.8 = 4.9 N. k = 4.9 ÷ 0.080 = 61.25 N/m, so 61 N/m.

4. A spring with k = 250 N/m is compressed by 4.0 cm. Find the force and the energy stored.

F = 250 × 0.040 = 10 N. E = ½ × 250 × 0.040² = 0.20 J.

5. A 0.40 kg mass on a spring makes 15 oscillations in 9.0 s. Find k.

T = 9.0 ÷ 15 = 0.60 s. k = 4π² × 0.40 ÷ 0.60² = 43.9 N/m, so 44 N/m.

6. A spring stores 1.25 J when stretched 5.0 cm. Find k.

k = 2 × 1.25 ÷ 0.050² = 1000 N/m (1.0 kN/m).

7. Springs of 150 N/m and 450 N/m are joined end to end. Find the equivalent spring constant. What if they are side by side?

Series: 150 × 450 ÷ 600 = 112.5 N/m, so 110 N/m. Parallel: 150 + 450 = 600 N/m.

8. A spring with k = 80 N/m is cut into two equal halves. What is the spring constant of each half?

The two halves in series make the original spring: 1/80 = 2/k, so each half has k = 160 N/m.

9. A car suspension spring is rated at 300 lbf/in. Convert to N/m and N/mm.

300 × 175.127 = 52 538 N/m, so 53 kN/m or 53 N/mm.

10. In an experiment, students calculate the spring constant as 2.5 N/m for a spring that stretched 4.0 cm under 100 g. What went wrong?

They divided the weight, 0.981 N, by 0.40 m, or the extension was taken as 40 cm. With x = 0.040 m, k = 0.981 ÷ 0.040 = 24.5 N/m, so 25 N/m. A factor of 10 or 100 almost always means a units slip.

14.6 Mistakes that lose marks

  • Dividing by the total length of the spring instead of the extension.
  • Leaving the extension in centimetres or millimetres.
  • Dividing the mass by the extension: you need the weight, m g.
  • Forgetting to square the period, or using 2π instead of 4π² after rearranging.
  • Adding spring constants for springs in series.
  • Swapping the axes on the graph, so the gradient is 1/k.
  • Giving k the unit N or N·m instead of N/m.

🎯 15. When to Use These Equations

Hooke’s law and the spring constant describe springs very well inside a clear range.

Use them when:

  • ✓ The force–extension graph is a straight line through the origin (or with a small offset for initial tension).
  • ✓ The spring returns to its original length when the load is removed.
  • ✓ The spring is light compared with the mass on it, for the period method.
  • ✓ The coils do not touch each other (for compression springs, they are not fully squashed).

Use something else when:

  • ✗ The spring is stretched past its limit of proportionality: the graph curves and k changes with the load.
  • ✗ The spring is heavy compared with the load: add about one third of the spring’s mass to m in the period formula.
  • ✗ The material is rubber or a bungee cord: its force–extension curve is not straight and shows hysteresis.
  • ✗ It is a constant-force spring (a coiled flat strip, as in a tape measure): its force hardly changes with extension, so it has no single k. Manufacturers give its rated force instead.
  • ✗ It is a conical or variable-pitch spring: k rises as coils bottom out, so use the manufacturer’s rate curve.

Real-world examples:

  1. Car suspension: the spring rate and the car’s mass set the ride frequency, f = (1/2π)√(k/m), usually about 1 to 2 Hz.
  2. Weighing scales and newton meters: the stretch is proportional to the load, so the scale can be marked evenly.
  3. Mattresses and seats: hundreds of springs in parallel share the load.
  4. Engine valves and door closers: a stiff compression spring returns the part quickly.
  5. Torsion springs in clothes pegs and mousetraps: the torque grows with the angle wound.
  6. Atoms and molecules: chemical bonds act like tiny springs, and their k sets the frequencies seen in infrared spectra.
Decision tree. Know the force and the stretch? k = F ÷ x. A hanging mass at rest? k = m g ÷ x. Only a stopwatch? k = 4π²m ÷ T². Know the stored energy? k = 2E ÷ x². Several springs? Add k in parallel, add 1/k in series. Designing a coil spring? k = G d⁴ ÷ (8 D³ n). Several readings? Plot F against x and take the gradient.

🔧 16. Common Mistakes and Troubleshooting

My k is 100 times too big

Cause: the extension was used in centimetres. Fix: pick cm in the unit menu or divide by 100. 2.00 N ÷ 4.00 cm is 50.0 N/m, not 0.500 N/m or 5000 N/m.

My k is about 10 times too small with a hanging mass

Cause: you divided the mass, not the weight, by the extension. Fix: k = m g ÷ x. For 0.200 kg and 4.90 cm, k = 1.962 ÷ 0.0490 = 40.0 N/m, not 4.08.

My answer used the whole length of the spring

Cause: Hooke’s law needs the change in length. Fix: choose “k from the two lengths” and enter both lengths; the calculator subtracts them.

The period method gives a smaller k than the static method

Cause: the spring’s own mass also oscillates. Fix: add about one third of the spring’s mass to m, or use the T² against m graph, whose gradient ignores it.

My series answer is bigger than both springs

Cause: you added k values, which is the rule for parallel springs. Fix: in series add the reciprocals: 1/k = 1/k₁ + 1/k₂.

"Not physical: the mean coil diameter must be larger than the wire"

Cause: D was entered smaller than d, often by typing the inner diameter in the wrong box or mixing mm and inches. Fix: mean coil diameter = outer diameter − d.

The graph gradient is about 0.025 instead of 40

Cause: the columns are swapped, so the fit gives extension per newton. Fix: put the extension first and the force (or mass) second in the Graph tab.

My graph has a large intercept

Cause: a zero error in the ruler reading, or initial tension in a closely wound extension spring. Fix: use the gradient, which is not affected by an offset.

The last digit differs from my textbook

Cause: a different g in a hanging-mass question. 9.8, 9.81 and 10 give 40.0, 40.0 and 40.8 N/m for 0.200 kg and 4.90 cm. Fix: set g to the value your question states.

The charts do not appear

Cause: the Chart.js library was blocked, or Calculate has not been pressed. Fix: press Calculate. The answer, steps, table and diagram work without the charts.

⚖ 17. Assumptions and Limitations

Assumptions

  1. Linear (Hookean) spring. Check: the force–extension readings lie on a straight line. Past the limit of proportionality k = F ÷ x underestimates how much the spring stretches.
  2. Massless spring. Check: the spring is much lighter than the load. A heavy spring makes the period method underestimate k unless about a third of its mass is added to m.
  3. Static readings at rest. Check: the mass has stopped bouncing before you read the extension.
  4. Small, undamped oscillations. Check: the amplitude dies away slowly. Strong damping lengthens the period slightly.
  5. Ideal series and parallel springs. In parallel the bar stays level and the load is centred; in series the joints are light.
  6. Round wire, uniform coils (dimensions mode). Only active coils count; ground ends are usually inactive. Real rates are typically within about 10% of the formula.
  7. Uniform g. g is taken as constant. Over the surface of the Earth it varies from 9.78 to 9.83 m/s², an effect of about 0.5%.

Limitations

  • It does not model non-linear springs such as rubber bands, conical springs or progressive-rate car springs.
  • It does not model constant-force springs, whose force is set by the strip thickness and coil radius rather than a k.
  • It does not include damping or forced vibration.
  • The dimensions mode gives the rate, not the stress; check stresses with a spring design standard before building a real spring.
  • Typical values in Section 12 are guides only; always measure or check the maker’s rating.

🏁 18. Conclusion

A spring constant calculator rests on one short law, F = k x, yet spring questions still lose marks every year. The division is easy. The hard parts are using the extension rather than the length, converting it to metres, and using the weight rather than the mass.

Three habits fix nearly every error. First, find x as the change in length and convert it to metres before you do anything else. Second, choose the right route to k: F ÷ x for a force, m g ÷ x for a hanging mass, 4π²m ÷ T² for a timed bounce, 2E ÷ x² for stored energy. Third, for real data, plot force against extension and take the gradient instead of trusting one reading.

For several springs, remember that they combine the opposite way to resistors: add k in parallel, add 1/k in series. For spring design, the rate depends on the fourth power of the wire diameter, so small changes in wire size make big changes in stiffness.

___ Press Calculate to add your own answer to this conclusion.

Pick your method, type your values in any units, and let this spring constant calculator show every step, a units check and a diagram drawn from your numbers.

❓ 19. Frequently Asked Questions

How do you calculate the spring constant?

Divide the force on the spring by its extension: k = F ÷ x, with x in metres. A force of 2.00 N that stretches a spring by 4.00 cm gives k = 2.00 ÷ 0.0400 = 50.0 N/m.

What is the formula for calculating the spring constant?

The spring constant formula comes from Hooke’s law, F = k x, so k = F ÷ x. Other forms are k = m g ÷ x for a hanging mass, k = 4π²m ÷ T² from the period and k = 2E ÷ x² from stored energy.

How do you calculate the spring constant with mass and distance?

Hang the mass, measure how far the spring stretches, and use k = m g ÷ x. A 0.200 kg mass that stretches a spring by 4.90 cm gives k = 0.200 × 9.81 ÷ 0.0490 = 40.0 N/m.

How do you calculate the spring constant from a graph?

Plot force on the y-axis against extension on the x-axis and draw the best-fit straight line. The gradient is k. If the axes are the other way round, the gradient is 1/k, so take the reciprocal.

How do you calculate the spring constant without force?

Time the oscillations of a known mass and use k = 4π²m ÷ T². A 0.500 kg mass with a period of 0.800 s gives k = 30.8 N/m. You can also use stored energy, k = 2E ÷ x², or the coil dimensions.

How do you calculate the spring constant from the period?

Rearrange T = 2π√(m ÷ k) to k = 4π²m ÷ T². Time 10 to 20 oscillations and divide by the count to get T. For 0.250 kg making 20 oscillations in 15.8 s, T = 0.790 s and k = 15.8 N/m.

How do you calculate the equivalent spring constant in series?

Add the reciprocals: 1/k = 1/k₁ + 1/k₂ + …. For two springs, k = k₁k₂ ÷ (k₁ + k₂). Springs of 200 N/m and 300 N/m in series give 120 N/m, softer than either.

How do you calculate the spring constant in parallel?

Add the spring constants: k = k₁ + k₂ + …. Springs of 200 N/m and 300 N/m side by side give 500 N/m, because they share the load and stretch by the same amount.

How do you calculate the effective spring constant?

The effective (equivalent) spring constant replaces several springs with one. Add k for springs in parallel and add 1/k for springs in series. For mixed systems, combine the inner groups first and work outward.

How do you calculate the spring force with the spring constant?

Multiply the spring constant by the extension: F = k x. A spring with k = 50.0 N/m stretched by 7.00 cm pulls back with 50.0 × 0.0700 = 3.50 N, toward its natural length.

How do you calculate the spring force without the spring constant?

Use the forces on the object instead. A mass hanging at rest has spring force F = m g. If it accelerates, F − m g = m a. If you know the stored energy and extension, F = 2E ÷ x.

How do you calculate the spring constant from dimensions?

For a helical compression or extension spring, k = G d⁴ ÷ (8 D³ n), where G is the shear modulus, d the wire diameter, D the mean coil diameter and n the active coils. Music wire with d = 2.00 mm, D = 20.0 mm and 10 coils gives 1.98 N/mm.

How do you calculate a torsion spring constant?

A torsion spring’s rate is a torque per angle: k = E d⁴ ÷ (64 D n) in N·m per radian, where E is Young’s modulus. Multiply by π/180 for N·m per degree. Steel wire of 2.00 mm, D = 15.0 mm and 6 coils gives about 0.575 N·m/rad.

How do you calculate the uncertainty of the spring constant?

For one reading, add the percentage uncertainties in F and x. For a graph, use the standard error of the gradient, or half the difference between the steepest and shallowest lines through the error bars. The sample data give 40.0 ± 0.3 N/m.

What is the unit of the spring constant?

The SI unit is the newton per metre (N/m), which equals kg/s². Spring makers often use N/mm (1 N/mm = 1000 N/m) or pounds-force per inch (1 lbf/in = 175.1 N/m).

What does a high spring constant mean?

A high k means a stiff spring that needs a large force to stretch or compress it a little. A low k means a soft spring. Stiffer springs also bounce faster for the same mass, because f is proportional to √k.

Does cutting a spring change its spring constant?

Yes. Cutting a spring in half doubles k, because the original spring was the two halves in series. In general, a piece containing a fraction 1/n of the coils has n times the spring constant.

How do you calculate the spring constant in Excel?

Put the force in B2 and the extension in metres in D2, then type =B2/D2. For a set of readings, use =SLOPE(force range, extension range) for the gradient and LINEST for its standard error.

How do you calculate the spring constant in Python?

Write k = F / x with SI values, then print(f"{k:.1f} N/m"). For lab data, use k, c = np.polyfit(extension, force, 1); the first number is the gradient. Section 11 has a full script that also saves a 300 dpi graph.

How do you calculate the spring constant in R?

Define F and x in SI units and run k <- F / x. For readings, fit <- lm(force ~ extension) and coef(fit)[2] is k, with its standard error in summary(fit). Section 10 has the full script.

📑 20. Cite This Tool

APA 7th edition
StatsUnlock Team. (2026). Spring constant calculator [Interactive physics tool]. StatsUnlock. https://statsunlock.com/spring-constant-calculator/
BibTeX
@misc{statsunlock_springconstant_2026, title={Spring Constant Calculator}, author={{StatsUnlock Team}}, year={2026}, note={Interactive physics tool}, url={https://statsunlock.com/spring-constant-calculator/}}
Methods wording
Spring constants were calculated with the StatsUnlock Spring Constant Calculator (2026) from Hooke’s law, k = F/x, with forces from hanging masses found using g = 9.81 m/s². For repeated readings, k was taken as the least-squares gradient of force against extension, with its standard error. Each result was checked by dimensional analysis and by substitution into the original equation.

📖 22. Glossary of Terms

TermPlain-English meaningSI unit
Active coils (n)The coils that actually flex; closed or ground ends do not count.-
Compliance1/k, how far the spring moves per newton.m/N
CompressionShortening of a spring under a push.metre (m)
Constant-force springA coiled flat strip whose force stays almost the same as it unrolls.-
Effective massThe part of the spring’s own mass that moves with the load, about one third of it.kilogram (kg)
Elastic limitThe largest load after which the spring still returns to its original length.-
Elastic potential energyEnergy stored in a stretched or compressed spring, ½ k x².joule (J)
Equivalent spring constantThe k of one spring that behaves like a group of springs.N/m
Extension (x)The increase in length from the natural length.metre (m)
Frequency (f)Oscillations per second, 1/T.hertz (Hz)
Hooke’s lawForce is proportional to extension, F = k x, within the limit of proportionality.-
Limit of proportionalityThe point where the force–extension graph stops being a straight line.-
Natural length (L₀)The length of the spring with no load.metre (m)
Parallel springsSprings side by side, sharing the load; k values add.-
Period (T)Time for one full oscillation.second (s)
Series springsSprings end to end, each carrying the full force; 1/k values add.-
Shear modulus (G)How strongly a material resists twisting; sets a coil spring’s rate.pascal (Pa)
Spring constant (k)Force per unit extension; also called the force constant, stiffness or spring rate.N/m
Spring index (C)Mean coil diameter divided by wire diameter, D/d.-
Torsion springA spring loaded by twisting; its rate is a torque per angle.N·m/rad

📚 23. References

These peer-reviewed articles support Hooke’s law, the period method and the spring’s effective mass, springs in series, graph fitting, the lab methods and the common mistakes covered by this spring constant calculator. Every DOI was checked in October 2026.

  1. Mohr, P. J., Newell, D. B., Taylor, B. N., & Tiesinga, E. (2025). CODATA recommended values of the fundamental physical constants: 2022. Reviews of Modern Physics, 97(2), 025002. https://doi.org/10.1103/RevModPhys.97.025002
  2. Moyer, A. E. (1977). Robert Hooke’s ambiguous presentation of “Hooke’s law”. Isis, 68(2), 266–275. https://doi.org/10.1086/351771
  3. Fox, J. G., & Mahanty, J. (1970). The effective mass of an oscillating spring. American Journal of Physics, 38(1), 98–100. https://doi.org/10.1119/1.1976240
  4. Leclerc, M. (1987). Effective elastic constant and effective mass of an oscillating spring: An energy approach. American Journal of Physics, 55(2), 178–180. https://doi.org/10.1119/1.15220
  5. Rodríguez, E. E., & Gesnouin, G. A. (2007). Effective mass of an oscillating spring. The Physics Teacher, 45(2), 100–103. https://doi.org/10.1119/1.2432087
  6. Galloni, E. E., & Kohen, M. (1979). Influence of the mass of the spring on its static and dynamic effects. American Journal of Physics, 47(12), 1076–1078. https://doi.org/10.1119/1.11978
  7. Gash, P. (2020). A Slinky’s elastic potential energy. The Physics Teacher, 58(3), 198–199. https://doi.org/10.1119/1.5145416
  8. Radomirovic, D., & Kovacic, I. (2015). An equivalent spring for nonlinear springs in series. European Journal of Physics, 36(5), 055004. https://doi.org/10.1088/0143-0807/36/5/055004
  9. Wulandari, S., Iswanto, B. H., & Sugihartono, I. (2021). Determination of springs constant by Hooke’s law and simple harmonic motion experiment. Journal of Physics: Conference Series, 2019(1), 012053. https://doi.org/10.1088/1742-6596/2019/1/012053
  10. Castro-Palacio, J. C., Velázquez-Abad, L., Giménez, M. H., & Monsoriu, J. A. (2013). Using a mobile phone acceleration sensor in physics experiments on free and damped harmonic oscillations. American Journal of Physics, 81(6), 472–475. https://doi.org/10.1119/1.4793438
  11. Vogt, P., & Kuhn, J. (2012). Analyzing simple pendulum phenomena with a smartphone acceleration sensor. The Physics Teacher, 50(7), 439–440. https://doi.org/10.1119/1.4752056
  12. Staacks, S., Hütz, S., Heinke, H., & Stampfer, C. (2018). Advanced tools for smartphone-based experiments: phyphox. Physics Education, 53(4), 045009. https://doi.org/10.1088/1361-6552/aac05e
  13. York, D. (1966). Least-squares fitting of a straight line. Canadian Journal of Physics, 44(5), 1079–1086. https://doi.org/10.1139/p66-090
  14. Brown, D. E., & Clement, J. (1989). Overcoming misconceptions via analogical reasoning: Abstract transfer versus explanatory model construction. Instructional Science, 18(4), 237–261. https://doi.org/10.1007/BF00118013
  15. Minstrell, J. (1982). Explaining the “at rest” condition of an object. The Physics Teacher, 20(1), 10–14. https://doi.org/10.1119/1.2340924
  16. Rosengrant, D., Van Heuvelen, A., & Etkina, E. (2009). Do students use and understand free-body diagrams? Physical Review Special Topics: Physics Education Research, 5(1), 010108. https://doi.org/10.1103/PhysRevSTPER.5.010108
  17. Heller, J. I., & Reif, F. (1984). Prescribing effective human problem-solving processes: Problem description in physics. Cognition and Instruction, 1(2), 177–216. https://doi.org/10.1207/s1532690xci0102_2
  18. Hestenes, D., Wells, M., & Swackhamer, G. (1992). Force concept inventory. The Physics Teacher, 30(3), 141–158. https://doi.org/10.1119/1.2343497
  19. Thornton, R. K., & Sokoloff, D. R. (1998). Assessing student learning of Newton’s laws: The Force and Motion Conceptual Evaluation and the evaluation of active learning laboratory and lecture curricula. American Journal of Physics, 66(4), 338–352. https://doi.org/10.1119/1.18863
  20. Hake, R. R. (1998). Interactive-engagement versus traditional methods: A six-thousand-student survey of mechanics test data for introductory physics courses. American Journal of Physics, 66(1), 64–74. https://doi.org/10.1119/1.18809
STATS UNLOCK · statsunlock.com · Spring Constant Calculator
RELATED ARTICLES

LEAVE A REPLY

Please enter your comment!
Please enter your name here

Most Popular

Statistical Models

Important Plots & Charts

Calculators and Converters