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.
⚡ 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.
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.
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 know | Spring constant formula | Key idea |
|---|---|---|
| Force and extension | k = F ÷ x | Hooke’s law, the definition of k |
| Natural and stretched length | k = F ÷ (L − L₀) | Use the change in length |
| Hanging mass and stretch | k = m g ÷ x | At rest the spring holds the weight |
| Period of oscillation | k = 4π² m ÷ T² | No ruler needed; time the bounce |
| Stored energy | k = 2E ÷ x² | Elastic potential energy, E = ½ k x² |
| Springs in series | 1/k = 1/k₁ + 1/k₂ | Softer than any one spring |
| Springs in parallel | k = k₁ + k₂ | Stiffer than any one spring |
| Coil dimensions | k = G d⁴ ÷ (8 D³ n) | Compression and extension springs |
| Lab graph | k = gradient of F against x | Best 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.
📊 3. Results and Step-by-Step Working
🧠 4. Understanding Your Answer, In Detail
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
📌 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.
📌 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.
📌 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.
📌 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.
📌 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
🔀 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.
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
- 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.
- 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.
- Type the known values. Plain numbers, decimals and scientific notation all work: 2.00, 0.0400 or 4e-2.
- Pick the unit beside each value. Grams, pounds, mm, cm, inches, lbf, N/mm, lbf/in, GPa and psi are converted for you.
- Use the change in length. Enter the extension or compression, or use the two-lengths option and let the calculator subtract.
- 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.
- Press Calculate. Nothing is worked out until you do, and changing any input clears the old answer so you never read stale numbers.
- Read the answer box and the badges. They show k with its units, other units, Units check ✓ and Verified.
- Follow the working, the diagram and the charts. The GRESS table, numbered steps, results table, diagram and four charts explain the answer.
- 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.
=B2/(C2/100). For 2.00 N and 4.00 cm, Excel returns 50, in N/m.9.1 The functions you need
| You want | Excel formula | Note |
|---|---|---|
| Extension in metres | =C2/100 | From cm; use /1000 for mm |
| Spring constant from force | =B2/D2 | k = F ÷ x with x in metres |
| Spring constant from a hanging mass | =B2/1000*$H$1/D2 | Mass in grams in B2, g in H1 |
| Spring constant from the period | =4*PI()^2*B2/C2^2 | Mass in kg in B2, period in s in C2 |
| Spring constant from energy | =2*B2/D2^2 | Energy in J in B2 |
| Two springs in series | =1/(1/B2+1/C2) | Add a +1/D2 for a third spring |
| Springs in parallel | =SUM(B2:D2) | k = k₁ + k₂ + k₃ |
| Gradient of a lab graph | =SLOPE(C2:C7,D2:D7) | Force column first, then extension |
| Standard error of the gradient | =INDEX(LINEST(C2:C7,D2:D7,TRUE,TRUE),2,1) | The ± on k |
| Coil spring rate | =B2*1E9*(C2/1000)^4/(8*(D2/1000)^3*E2) | G in GPa, d and D in mm, n coils |
9.2 Step by step
1 Set up the columns
One spring per row, units in every header. Type the extension as you measured it, in centimetres.
Three springs in rows 2 to 4. Column D will hold the extension in metres.
2 Convert the extension to metres
=C2/100Hooke’s law needs SI units. Fill the formula down to row 4.
4.00 cm becomes 0.04 m.
3 Divide force by extension
=B2/D2This is k = F ÷ x. Fill it down.
The lab spring gives 50 N/m; format the cell to one decimal place to show 50.0.
4 Find k from a graph of readings
=SLOPE(C2:C7,D2:D7)On a second sheet, put the masses in B (g), the force in C (=B2/1000*$H$1) and the extension in metres in D. SLOPE gives the gradient.
Six readings give k = 40.0 N/m, with a standard error of 0.32 N/m from LINEST.
9.3 The awkward cases
=SLOPE(D2:D7,C2:C7) gives extension per newton, 0.025 m/N, the reciprocal of k. Put the force column first so the answer is in N/m.- Initial tension. A closely wound extension spring needs some force before it stretches at all. Its line does not pass through zero, so use
SLOPE, not a single F ÷ x. - Two lengths. If you recorded lengths, not extensions, make a column
=C2-$C$2to 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")*0.0254for 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 g | k for 0.200 kg stretching 4.90 cm | Who uses it |
|---|---|---|
| 9.80665 m/s² | 40.027 N/m → 40.0 N/m | Standard gravity; scipy.constants.g in Python |
| 9.81 m/s² | 40.041 N/m → 40.0 N/m | IGCSE, A level, most US textbooks, this page |
| 9.8 m/s² | 40.0 N/m | CBSE, NCERT, AP Physics |
| 10 m/s² | 40.8 N/m | Quick 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
- Put the extension (m) in one column and the force (N) in the next.
- Insert → Charts → Scatter. Do not use the Line chart type, which spaces x values evenly.
- Click a point → Add Trendline → Linear → tick Display Equation on chart and Display R-squared.
- The number in front of x in the equation is k. An intercept far from zero points to a zero error or initial tension.
- Label the axes with units: Extension x (m) and Force F (N).
9.6 Errors you will hit
| Excel shows | Why | Fix |
|---|---|---|
#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 blank | Fill the cell, or wrap with =IFERROR(...,"") |
| k is 100 times too small | Extension left in cm, divided by 100 twice, or k divided by 100 | Convert once, in its own column |
| k is 100 times too big | Extension used in cm instead of m | Divide the cm value by 100 |
| SLOPE answer is about 0.025 | x and y swapped | Force range first: =SLOPE(force, extension) |
| Every row gives the same answer | Relative reference to g changed when you filled down | Use $H$1, not H1 |
📈 10. How to Calculate the Spring Constant in R
R works out the spring constant in one line, k <- F / x. The script below does more: it finds k from a hanging mass, the period, stored energy, series and parallel springs and coil dimensions, fits a straight line to lab readings with lm() for the gradient and its standard error, runs a substitution check, and saves one 300 dpi figure of force against extension. It uses base R only.
Rscript sk.R. Change only the lines under "Your values" and the two data vectors to solve your own problem.10.1 The complete script
R, base only, saves spring_constant_plot.png
# ============================================================
# Spring constant calculator in R, with one 300 dpi figure
# StatsUnlock - https://statsunlock.com/spring-constant-calculator/
# Base R only. No packages to install.
# ============================================================
# ---- 1. Your values (SI units) -----------------------------
F <- 2.00 # force on the spring in N
x <- 4.00 / 100 # extension in m (4.00 cm)
g <- 9.81 # m/s^2; use 9.80665, 9.8 or 10 to match your book
# ---- 2. Hooke's law: k = F / x -----------------------------
k <- F / x # N/m
# ---- 3. Other ways to find k -------------------------------
k_mass <- 0.200 * g / (4.90 / 100) # hanging mass 0.200 kg, stretch 4.90 cm
k_period <- 4 * pi^2 * 0.500 / 0.800^2 # 0.500 kg bouncing with T = 0.800 s
k_energy <- 2 * 0.90 / (6.0 / 100)^2 # 0.90 J stored at 6.0 cm
k_series <- 1 / (1 / 200 + 1 / 300) # 200 N/m and 300 N/m end to end
k_par <- 200 + 300 # the same springs side by side
k_coil <- 79.3e9 * (2.00e-3)^4 / (8 * (20.0e-3)^3 * 10) # G d^4 / (8 D^3 n)
# ---- 4. From a graph: least-squares gradient ---------------
ext <- c(0, 2.4, 5.0, 7.3, 9.9, 12.2) / 100 # extension, m
load <- c(0, 100, 200, 300, 400, 500) / 1000 * g # weight of the masses, N
fit <- lm(load ~ ext)
k_graph <- unname(coef(fit)[2])
se <- summary(fit)$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
| Line | What it does |
|---|---|
| x <- 4.00 / 100 | Converts 4.00 cm to metres before anything else. |
| k <- F / x | Hooke’s law rearranged: spring constant in N/m. |
| 4 * pi^2 * 0.500 / 0.800^2 | k 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
| Problem | Fix |
|---|---|
| k is 100 times too big | The extension was in centimetres. Divide by 100. |
| Gradient is about 0.025 | The formula is backwards. Use lm(force ~ extension). |
| The PNG is blank | dev.off() did not run; run the last line again. |
| Later code using F as FALSE breaks | Rename 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.
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
| Line | What it does |
|---|---|
| k = F / x | Hooke’s law rearranged, N/m. |
| 4 * np.pi**2 * 0.500 / 0.800**2 | k 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
| Problem | Fix |
|---|---|
| ModuleNotFoundError: numpy | Run pip install numpy matplotlib, or use Google Colab. |
| k is 100 times too big | Divide extensions in cm by 100 before fitting. |
| No PNG appears | Look in the folder you ran the script from, or print os.getcwd(). |
| Last digit differs from your textbook | Set 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 know | Spring constant | Unit check |
|---|---|---|
| Force F and extension x | F ÷ x | N ÷ m |
| Force and two lengths | F ÷ (L − L₀) | N ÷ m |
| Hanging mass m and stretch x | m g ÷ x | kg × m/s² ÷ m |
| Mass and period T | 4π² m ÷ T² | kg ÷ s² |
| Mass and frequency f | 4π² m f² | kg × Hz² |
| n oscillations in time t | 4π² m n² ÷ t² | kg ÷ s² |
| Energy E and extension x | 2E ÷ x² | J ÷ m² |
| Springs in series | 1 ÷ (1/k₁ + 1/k₂ + …) | N/m |
| Springs in parallel | k₁ + k₂ + … | N/m |
| A spring cut into n equal pieces | each piece n k | N/m |
| Coil dimensions | G d⁴ ÷ (8 D³ n) | Pa × m |
| Torsion spring | E d⁴ ÷ (64 D n) | N·m per radian |
12.2 Typical spring constants
| Spring | Typical k | Note |
|---|---|---|
| Slinky toy | about 1 N/m | Very soft |
| School lab spring | 5 to 100 N/m | Stretches several cm under 100 g masses |
| Ballpoint pen spring | 100 to 500 N/m | Small and stiff |
| Trampoline spring | about 5 kN/m | Each of many in parallel |
| Car suspension spring | 20 to 100 kN/m | Often quoted as 20 to 100 N/mm |
| Engine valve spring | 30 to 100 kN/m | Very stiff, short travel |
Typical published ranges; real springs vary widely. Measure your own spring when accuracy matters.
12.3 Spring wire materials
| Material | Shear modulus G | Young’s modulus E |
|---|---|---|
| Music wire (high-carbon steel) | 79.3 GPa (11.5 Mpsi) | 207 GPa (30 Mpsi) |
| Stainless steel 302 | 69.0 GPa (10.0 Mpsi) | 193 GPa (28 Mpsi) |
| Chrome silicon | 77.2 GPa (11.2 Mpsi) | 203 GPa (29.5 Mpsi) |
| Beryllium copper | 48.3 GPa (7.0 Mpsi) | 128 GPa (18.5 Mpsi) |
| Phosphor bronze | 41.4 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
| Unit | In N/m | Where you meet it |
|---|---|---|
| 1 N/cm | 100 N/m | School practicals |
| 1 N/mm | 1000 N/m | Spring catalogues, car suspension |
| 1 kN/m | 1000 N/m | Engineering |
| 1 lbf/in | 175.127 N/m | US spring catalogues, coilovers |
| 1 kgf/mm | 9806.65 N/m | Japanese and European suspension specs |
12.5 Constants
| Constant | Value | Status |
|---|---|---|
| Standard acceleration of gravity g₀ | 9.806 65 m/s² | exact (defined) |
| Typical surface g on Earth | 9.78 (equator) to 9.83 (poles) m/s² | measured |
| 4π² | 39.478 4 | mathematical constant |
| 1 lbf | 4.448 222 N | exact by definition |
Source: NIST, CODATA 2022 recommended values (Mohr et al., 2025, reference 1).
📈 13. Example Results
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.
| Quantity | Value | Note |
|---|---|---|
| 1 lbf/in in N/m | 175.127 N/m | 4.448 N ÷ 0.0254 m |
| k in N/m | 78,807 N/m | 450 × 175.127 |
| F = k x | 900 lbf = 4,003 N | 450 × 2.0 in |
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."
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.
| Quantity | Value | Note |
|---|---|---|
| k = F ÷ x | 10.0 ÷ 0.0800 = 125 N/m | 3 s.f. |
| x = F ÷ k at 3.5 N | 0.028 m = 2.8 cm | Linear scale |
| Scale spacing | 0.800 cm per newton | 1 ÷ k |
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."
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.
| Quantity | Value | Note |
|---|---|---|
| Spring force k x | 1.5 N | 25 × 0.060 |
| m = k x ÷ g | 0.1531 kg → 0.15 kg | 2 s.f. |
| In grams | 150 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."
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.
| Quantity | Value | Note |
|---|---|---|
| k = k₁ + k₂ + k₃ + k₄ | 120 000 N/m | Parallel springs add |
| ω = √(k ÷ m) | 10.0 rad/s | √(120 000 ÷ 1200) |
| f = ω ÷ 2π | 1.592 Hz → 1.6 Hz | Comfortable ride: 1 to 1.5 Hz |
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."
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.
| Quantity | Value | Note |
|---|---|---|
| Convert | 2.0 × 10⁻⁸ m | 20 × 10⁻⁹ |
| F = k x | 2.0 × 10⁻⁹ N | 2 s.f. |
| In nN | 2.0 nN |
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."
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.
| Quantity | Value | Note |
|---|---|---|
| Gradient, first 5 points | 49.99 N/m → 50 N/m | Straight part only |
| Expected x at 6 N | 12.0 cm | Measured 13.5 cm |
| Verdict | Past the limit of proportionality | Do not load further |
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."
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?
| Quantity | Value | Note |
|---|---|---|
| Original = two halves in series | 1/80 = 1/k + 1/k | |
| Solve | k = 2 × 80 = 160 N/m | |
| One third of the coils | 240 N/m | n pieces: n k |
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."
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.
| Quantity | Value | Note |
|---|---|---|
| x = L − L₀ | 2.5 cm = 0.025 m | Change in length |
| k = F ÷ x | 0.50 ÷ 0.025 = 20 N/m | Correct |
| F ÷ L (wrong) | 0.50 ÷ 0.125 = 4.0 N/m | Used total length |
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)
- Given. F = 2.00 N, x = 4.00 cm = 0.0400 m.
- Required. Spring constant k (N/m), then the force for x = 7.00 cm.
- Equation. Hooke’s law, F = k x, so k = F ÷ x.
- Substitute, with units. k = 2.00 N ÷ 0.0400 m. Then F = k × 0.0700 m.
- Solve. k = 50.0 N/m. F = 50.0 × 0.0700 = 3.50 N.
- 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.
- Write the answer with units. k = 50.0 N/m; F = 3.50 N.
14.2 On a scientific calculator
| Task | Keys | What you see |
|---|---|---|
| Force and extension | 2 ÷ 0.04 = | 50 |
| Hanging mass and stretch | 0.2 × 9.81 ÷ 0.049 = | 40.0408... |
| From the period | 4 × 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 B | 40.007... |
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
- 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.
- 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.
- 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.
- Calculate the force on the spring for each load, F = m g, with m in kilograms.
- Plot force (y-axis) against extension (x-axis) in metres. Draw the best-fit straight line through the origin. Its gradient is k.
- 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) |
|---|---|---|---|---|
| 100 | 0.981 | 2.4 | 0.024 | 40.9 |
| 200 | 1.962 | 5.0 | 0.050 | 39.2 |
| 300 | 2.943 | 7.3 | 0.073 | 40.3 |
| 400 | 3.924 | 9.9 | 0.099 | 39.6 |
| 500 | 4.905 | 12.2 | 0.122 | 40.2 |
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:
- 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.
- Weighing scales and newton meters: the stretch is proportional to the load, so the scale can be marked evenly.
- Mattresses and seats: hundreds of springs in parallel share the load.
- Engine valves and door closers: a stiff compression spring returns the part quickly.
- Torsion springs in clothes pegs and mousetraps: the torque grows with the angle wound.
- Atoms and molecules: chemical bonds act like tiny springs, and their k sets the frequencies seen in infrared spectra.
🔧 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
- 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.
- 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.
- Static readings at rest. Check: the mass has stopped bouncing before you read the extension.
- Small, undamped oscillations. Check: the amplitude dies away slowly. Strong damping lengthens the period slightly.
- Ideal series and parallel springs. In parallel the bar stays level and the load is centred; in series the joints are light.
- 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.
- 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.
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
StatsUnlock Team. (2026). Spring constant calculator [Interactive physics tool]. StatsUnlock. https://statsunlock.com/spring-constant-calculator/@misc{statsunlock_springconstant_2026, title={Spring Constant Calculator}, author={{StatsUnlock Team}}, year={2026}, note={Interactive physics tool}, url={https://statsunlock.com/spring-constant-calculator/}}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
| Term | Plain-English meaning | SI unit |
|---|---|---|
| Active coils (n) | The coils that actually flex; closed or ground ends do not count. | - |
| Compliance | 1/k, how far the spring moves per newton. | m/N |
| Compression | Shortening of a spring under a push. | metre (m) |
| Constant-force spring | A coiled flat strip whose force stays almost the same as it unrolls. | - |
| Effective mass | The part of the spring’s own mass that moves with the load, about one third of it. | kilogram (kg) |
| Elastic limit | The largest load after which the spring still returns to its original length. | - |
| Elastic potential energy | Energy stored in a stretched or compressed spring, ½ k x². | joule (J) |
| Equivalent spring constant | The 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 law | Force is proportional to extension, F = k x, within the limit of proportionality. | - |
| Limit of proportionality | The 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 springs | Springs side by side, sharing the load; k values add. | - |
| Period (T) | Time for one full oscillation. | second (s) |
| Series springs | Springs 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 spring | A 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.
- 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
- Moyer, A. E. (1977). Robert Hooke’s ambiguous presentation of “Hooke’s law”. Isis, 68(2), 266–275. https://doi.org/10.1086/351771
- 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
- 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
- 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
- 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
- Gash, P. (2020). A Slinky’s elastic potential energy. The Physics Teacher, 58(3), 198–199. https://doi.org/10.1119/1.5145416
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- Hestenes, D., Wells, M., & Swackhamer, G. (1992). Force concept inventory. The Physics Teacher, 30(3), 141–158. https://doi.org/10.1119/1.2343497
- 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
- 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
