BMI Calculator
Free body mass index calculator for men, women, children and teens. Get your BMI, category, healthy weight range and body fat estimate, with Asian and Japanese cut-offs, a CDC percentile mode for ages 2 to 19, an adjusted BMI for amputees, and a reverse BMI calculator.
By the StatsUnlock Team · Last updated: · Formula: WHO (adult), CDC 2000 LMS (child), Osterkamp 1995 (amputee)
🧮 1. BMI Calculator
Inputs
Missing limb segments. Enter your current measured weight above. The tool estimates what you would weigh with all limbs, then calculates BMI from that adjusted weight.
⚙ Advanced options: waist, activity, athlete
Your BMI
___
Calculate first, then download your result as a plain text file.
📊 2. Your BMI Results and BMI Chart
Your formula, with your numbers
Full results table
Where you sit in the categories
BMI Chart: What Is a Healthy Weight for My Height?
These reference tables apply to everyone. After you calculate, the green table switches to your height and highlights your row.
Table 2.1 BMI classification with BMI Prime (WHO adult)
| Classification | BMI (kg/m²) | BMI Prime | Weight at 175 cm | Health risk |
|---|---|---|---|---|
| Underweight Severe thinness | Below 16 | < 0.64 | Below 49.0 kg | High |
| Underweight Moderate thinness | 16.0 to 16.9 | 0.64 to 0.67 | 49.0 to 51.8 kg | Raised |
| Underweight Mild thinness | 17.0 to 18.4 | 0.68 to 0.73 | 52.1 to 56.4 kg | Slightly raised |
| Healthy weight Normal range | 18.5 to 24.9 | 0.74 to 0.99 | 56.7 to 76.3 kg | Lowest average |
| Overweight Pre-obese | 25.0 to 29.9 | 1.00 to 1.19 | 76.6 to 91.6 kg | Increased |
| Obese Obesity class I | 30.0 to 34.9 | 1.20 to 1.39 | 91.9 to 106.9 kg | Moderate |
| Obese Obesity class II | 35.0 to 39.9 | 1.40 to 1.59 | 107.2 to 122.2 kg | Severe |
| Obese Obesity class III | 40.0 and above | 1.60 and above | 122.5 kg and above | Very severe |
Source: World Health Organization adult BMI classification, in use as of 2026 (see the WHO obesity and overweight fact sheet). BMI Prime is BMI divided by 25, the upper limit of a healthy BMI, so a Prime of 1.00 sits exactly on the overweight line. Weights are shown for a 175 cm adult until you run the calculator, then they switch to your height and your row is highlighted. What to conclude: the healthy band is 6.4 BMI points wide, so most adults can be healthy across a weight span of 15 to 20 kg.
Table 2.2 Asian, Japanese and ethnicity-specific BMI cut-offs
| Standard | Healthy | Overweight | Obese | Who it is for |
|---|---|---|---|---|
| WHO international | 18.5 to 24.9 | 25.0 to 29.9 | 30.0+ | Most adults |
| WHO Asian (2004) | 18.5 to 22.9 | 23.0 to 27.4 | 27.5+ | South Asian, Chinese, Southeast Asian adults |
| Japan (JASSO) | 18.5 to 24.9 | Not used | 25.0+ (obesity grade 1 to 4) | Adults in Japan |
| NICE lower thresholds (UK) | 18.5 to 22.9 | 23.0 to 27.4 | 27.5+ | South Asian, Chinese, other Asian, Middle Eastern, Black African, African-Caribbean |
Sources: WHO Expert Consultation (2004); Japan Society for the Study of Obesity (2002); NICE guideline NG246, as of 2026. The NHS BMI calculator applies the same lower thresholds. Confirm current national guidance with your health service. What to conclude: the same BMI of 24 is healthy under WHO but overweight under the Asian and NICE thresholds, because diabetes risk rises at a lower BMI in these groups.
Table 2.3 BMI chart: healthy weight by height
| Height | Feet and inches | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|---|
| 150 cm | 4 ft 11 in | 41.6 to 56.0 kg | 92 to 124 lb |
| 155 cm | 5 ft 1 in | 44.4 to 59.8 kg | 98 to 132 lb |
| 160 cm | 5 ft 3 in | 47.4 to 63.7 kg | 104 to 141 lb |
| 165 cm | 5 ft 5 in | 50.4 to 67.8 kg | 111 to 149 lb |
| 170 cm | 5 ft 7 in | 53.5 to 72.0 kg | 118 to 159 lb |
| 175 cm | 5 ft 9 in | 56.7 to 76.3 kg | 125 to 168 lb |
| 180 cm | 5 ft 11 in | 59.9 to 80.7 kg | 132 to 178 lb |
| 185 cm | 6 ft 1 in | 63.3 to 85.2 kg | 140 to 188 lb |
| 190 cm | 6 ft 3 in | 66.8 to 89.9 kg | 147 to 198 lb |
| 195 cm | 6 ft 5 in | 70.3 to 94.7 kg | 155 to 209 lb |
Weights are for BMI 18.5 and 24.9 at each height, rounded to 0.1 kg and 1 lb. What to conclude: every 5 cm of height adds about 4 to 5 kg to the top of your healthy range.
Table 2.4 BMI chart for men and women: weight at each BMI line
| Height | BMI 18.5 (kg) | BMI 25 (kg) | BMI 30 (kg) | BMI 35 (kg) | BMI 40 (kg) |
|---|---|---|---|---|---|
| 150 cm (4 ft 11 in) | 41.6 | 56.3 | 67.5 | 78.8 | 90.0 |
| 155 cm (5 ft 1 in) | 44.4 | 60.1 | 72.1 | 84.1 | 96.1 |
| 160 cm (5 ft 3 in) | 47.4 | 64.0 | 76.8 | 89.6 | 102.4 |
| 165 cm (5 ft 5 in) | 50.4 | 68.1 | 81.7 | 95.3 | 108.9 |
| 170 cm (5 ft 7 in) | 53.5 | 72.3 | 86.7 | 101.2 | 115.6 |
| 175 cm (5 ft 9 in) | 56.7 | 76.6 | 91.9 | 107.2 | 122.5 |
| 180 cm (5 ft 11 in) | 59.9 | 81.0 | 97.2 | 113.4 | 129.6 |
| 185 cm (6 ft 1 in) | 63.3 | 85.6 | 102.7 | 119.8 | 136.9 |
| 190 cm (6 ft 3 in) | 66.8 | 90.3 | 108.3 | 126.4 | 144.4 |
| 195 cm (6 ft 5 in) | 70.3 | 95.1 | 114.1 | 133.1 | 152.1 |
The same chart applies to men and women, because the adult formula and cut-offs do not change with sex. What to conclude: read across your height row to see exactly which weight would move you into each category.
Table 2.5 CDC BMI percentile cut-offs for children and teens
| Age | Boys: BMI at percentile | Girls: BMI at percentile | ||||
|---|---|---|---|---|---|---|
| 5th | 85th | 95th | 5th | 85th | 95th | |
| 2 years | 14.7 | 18.1 | 19.3 | 14.4 | 18.0 | 19.1 |
| 4 years | 14.0 | 16.9 | 17.8 | 13.7 | 16.8 | 18.0 |
| 6 years | 13.7 | 17.0 | 18.4 | 13.4 | 17.1 | 18.8 |
| 8 years | 13.8 | 18.0 | 20.1 | 13.5 | 18.3 | 20.7 |
| 10 years | 14.2 | 19.4 | 22.2 | 14.0 | 20.0 | 23.0 |
| 12 years | 15.0 | 21.0 | 24.2 | 14.8 | 21.7 | 25.3 |
| 14 years | 16.0 | 22.7 | 26.0 | 15.8 | 23.3 | 27.3 |
| 16 years | 17.1 | 24.2 | 27.6 | 16.8 | 24.7 | 28.9 |
| 18 years | 18.2 | 25.7 | 29.0 | 17.6 | 25.7 | 30.3 |
Computed from the CDC 2000 BMI-for-age LMS table at the half-month row for each birthday. Categories: under 5th underweight, 5th to under 85th healthy, 85th to under 95th overweight, 95th and above obesity. Source: CDC growth charts, as of 2026. What to conclude: a BMI of 20 is in the obesity range for a 6-year-old but healthy for a 16-year-old, which is why children need percentiles.
Table 2.6 Body weight by limb segment, for adjusted BMI in amputees
| Segment missing | % of total body weight | Divide measured weight by |
|---|---|---|
| Hand | 0.7 | 0.993 |
| Forearm with hand (below elbow) | 2.3 | 0.977 |
| Entire arm | 5.0 | 0.950 |
| Foot | 1.5 | 0.985 |
| Lower leg with foot (below knee) | 5.9 | 0.941 |
| Entire leg | 16.0 | 0.840 |
| Both legs below knee | 11.8 | 0.882 |
Source: Osterkamp (1995). What to conclude: a person with an above-knee amputation who weighs 63 kg would weigh about 75 kg with the full leg, so an unadjusted BMI would read about 3.9 points too low at 175 cm.
Table 2.7 Underweight grades and when to seek help
| BMI | WHO thinness grade | What it usually means |
|---|---|---|
| 17.0 to 18.4 | Grade 1, mild | Check diet, appetite and recent weight change with a GP |
| 16.0 to 16.9 | Grade 2, moderate | Medical review recommended |
| Below 16.0 | Grade 3, severe | Needs prompt medical care |
Clinicians also use adult BMI to grade the severity of anorexia nervosa (mild 17 and above, moderate 16 to 16.99, severe 15 to 15.99, extreme below 15), but a diagnosis depends on behaviour and thoughts as well as weight. If you are worried about eating, you deserve support at any BMI. In the US, the National Alliance for Eating Disorders helpline is (866) 662-1235. Elsewhere, start with your doctor. What to conclude: a low BMI is a reason to talk to someone, not a target.
Table 2.8 BMI thresholds for bariatric (weight-loss) surgery
| Group | BMI that qualifies for assessment |
|---|---|
| Adults | 35 or above, with or without related disease |
| Adults with type 2 diabetes or other metabolic disease | 30 to 34.9, if other treatment has not worked |
| Asian adults | 27.5 or above may be considered |
| Teenagers | 120% of the 95th percentile with a major related condition, or 140% of the 95th percentile |
Source: ASMBS and IFSO indications (Eisenberg et al., 2022), as of 2026. Insurers and national health services may use stricter rules. What to conclude: BMI opens the door to a surgical assessment. It does not decide surgery on its own.
Visualisations
1. Your Weight vs Healthy Range
Your weight beside the lowest and highest healthy weight for your height
2. BMI Across Weights at Your Height
The line shows BMI at each weight. Dashed lines mark the category cut-offs
⚡ Quick Answer: How Do You Calculate BMI?
A BMI calculator divides your weight in kilograms by your height in metres squared.
Adults: under 18.5 is underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 or more obese. In US units, use 703 × lb ÷ in².
Rule of thumb: for Asian adults, 23 counts as overweight. Children use CDC percentiles.
Key Takeaways
- A BMI calculator uses one formula for everyone: weight in kg divided by height in metres squared.
- The adult healthy range is 18.5 to 24.9, and obesity starts at 30.
- For Asian adults, overweight starts at 23 and obesity at 27.5.
- Children aged 2 to 19 are judged by CDC percentile: the 85th marks overweight and the 95th marks obesity.
- Amputees should use an adjusted BMI, because a below-knee amputation alone removes about 5.9% of body weight.
- A waist-to-height ratio under 0.5 is a useful second check, especially for muscular people.
Reviewed for accuracy against WHO, CDC and peer-reviewed sources. Last updated: .
📚 3. What Is Body Mass Index (BMI)?
Body mass index (BMI) is a number that compares your weight with your height. It equals your weight in kilograms divided by the square of your height in metres. Doctors use it as a quick first screen for underweight, overweight and obesity in adults.
Every BMI calculator, including this one, uses the same core formula. The idea is old. A Belgian scientist, Adolphe Quetelet, noticed in the 1830s that adult weight rises roughly with the square of height. In 1972, Ancel Keys and colleagues tested several weight-for-height indices and named this one the body mass index, because it tracked body fat better than the others they tried. The World Health Organization (WHO) later set the adult cut-offs that most countries use today.
A worked one-liner: a person who weighs 70 kg and stands 175 cm tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 kg/m². That sits in the healthy range.
Who uses BMI, and why?
Family doctors, the NHS, the CDC, insurers and researchers all use BMI because it needs only a scale and a tape, it costs nothing, and it is the same everywhere. Large studies link high and very low BMI with higher risk of type 2 diabetes, heart disease and early death. That makes it a sensible first question. It is not the last one.
BMI compared with other body measures
| Measure | What it needs | What it tells you | Main weakness |
|---|---|---|---|
| BMI | Height, weight | Weight relative to height | Cannot tell muscle from fat |
| Waist-to-height ratio | Height, tape measure | Belly fat, linked to heart and diabetes risk | Waist must be measured carefully |
| Body fat percentage | Scan, calipers or scale | How much of you is fat | Home scales vary by 3 to 5 points |
| BMI-for-age percentile | Height, weight, age, sex | A child's BMI compared with peers | Needs exact age in months |
| Adjusted BMI (amputee) | Height, weight, limb loss | BMI corrected for missing limb weight | Uses average segment weights |
What to conclude: BMI is the best first screen because it is simple and universal. Pair it with waist-to-height ratio when muscle or body shape could mislead.
🔍 4. What Does Your BMI Result Mean?
4.1 What your number means
Your BMI of ___ puts you in the ___ category. Run the calculator above to fill this in.
4.2 How far you are from the healthy range
At your height, a healthy weight is ___ to ___. Run the calculator above.
4.3 What the BMI categories mean for health
Underweight (below 18.5) is linked with weak bones, low immunity and fertility problems. The healthy range (18.5 to 24.9) has the lowest average risk in large studies. Overweight (25 to 29.9) raises the risk of high blood pressure and type 2 diabetes. Obesity (30 and above) raises those risks further and adds sleep apnoea, joint pain and some cancers. Risk climbs gradually. Nothing magic happens at 25.0.
4.4 Your result under other standards
Your BMI of ___ would be labelled ___ under WHO, ___ under the Asian cut-offs and ___ under the Japanese standard. Run the calculator above.
4.5 Your waist, body fat and energy needs
Estimated body fat ___%, waist-to-height ratio ___, maintenance calories ___ kcal a day. Run the calculator above.
4.6 Why BMI can be wrong for you
BMI counts every kilogram the same, whether it is muscle, bone, water or fat. A rugby player and a desk worker can share a BMI of 28 and have very different health. BMI also tends to underrate fat in older adults, who lose muscle as they age, and in many Asian adults, who carry more body fat at the same BMI. Pregnancy makes BMI meaningless because the baby, fluid and placenta add weight.
4.7 What to do next
Your next step depends on your category. Run the calculator above for a personal plan.
4.8 Common mistakes this BMI calculator prevents
- Using adult cut-offs on a child. The Child & Teen tab switches to CDC percentiles automatically.
- Ignoring ethnicity. You can switch to Asian, Japanese or NICE thresholds, which flag risk at a lower BMI.
- Forgetting a missing limb. The Amputee tab corrects weight, so BMI is not falsely low.
- Mixing units. Switching units converts your numbers, so 175 cm never becomes 175 inches.
- Unsafe calorie targets. The calorie plan never goes below 1,500 kcal for men or 1,200 kcal for women, and never shows a weight-loss plan when your BMI is already under 18.5.
- Rounding at the edge. The category uses the same one-decimal BMI you see, so 24.96 shows as 25.0 and is labelled overweight, with no mismatch between number and label.
4.9 What the StatsUnlock team recommends
Read your BMI as a range, not a point. Normal daily swings in water and food move weight by 1 to 2 kg, which is 0.3 to 0.7 BMI points for most adults. So a single reading of 25.0 tells you far less than three readings averaged over a month. Pick the right standard before you react: for a South Asian woman of 165 cm, the top of the healthy range is 62.3 kg on the Asian scale and 67.8 kg on the WHO scale, a 5.4 kg difference from the same BMI calculator. And if you lift weights, trust your waist-to-height ratio over your BMI category.
📝 5. How to Use Your BMI Calculator Result
The BMI calculator builds five ready-made notes from your numbers. Copy the one you need.
1. Plain summary to save or share
2. Questions to ask your doctor
3. Goal plan
4. Tracking log (re-measure every 4 weeks)
5. Formal wording for a report or form
∑ 6. BMI Formulas Used (Formula of BMI, Body Mass Index)
👉 7. How to Use This BMI Calculator
Using this BMI calculator takes under a minute. The worked example below runs through every step: a 30-year-old man, 175 cm tall, 70 kg.
- Pick a mode. Choose Adult BMI for ages 18 and over, Child & Teen for ages 2 to 19, Amputee for limb loss, or Reverse BMI to find a target weight. The example uses Adult BMI.
- Choose your units. Pick a height unit (cm, metres, feet and inches, or inches) and a weight unit (kg, pounds, or stone and pounds). Switching converts any numbers already typed, so nothing is lost.
- Enter sex and age. Pick male, and type 30. Sex and age do not change BMI, but they drive the body fat and calorie estimates.
- Enter your height. Type 175 in the centimetre box, measured without shoes against a wall.
- Enter your weight. Type 70 in the kilogram box, ideally from a morning weigh-in.
- Pick a cut-off standard. Leave WHO international for most adults, or choose Asian, Japanese or NICE thresholds if they apply to you.
- Add advanced options. Optionally open Advanced options to add a waist measurement, your activity level and an athlete flag.
- Press Calculate BMI. The result panel shows 22.9 kg/m2, Healthy weight, and a healthy range of 56.7 to 76.3 kg.
- Read Section 2. Scroll to Your BMI Results for body fat (18.1%), BMR (1,649 kcal), maintenance calories, two charts and a category table.
- Save or share. Use Download TXT, Download Doc or Download PDF, or copy one of the five ready-made notes in Section 5.
📈 8. How to Calculate BMI in Excel
Excel has no BMI function, but you only need one short formula. The parts that trip people up are the unit conversion (centimetres to metres), rounding at category edges, and turning the number into a category label. Every step below uses the same worked example as the BMI calculator above: 70 kg and 175 cm, so you can check Excel against it.
=A2/(B2/100)^2 and press Enter. The answer is 22.857, which rounds to 22.9.8.1 The functions you need
| You want | Excel formula | Note |
|---|---|---|
| BMI, metric | =A2/(B2/100)^2 | Weight in kg, height in cm |
| BMI, US units | =703*A2/B2^2 | Weight in lb, height in inches |
| Round to 1 decimal | =ROUND(C2,1) | Classify on this value, not the raw one |
| Category label | =IFS(D2<18.5,"Underweight",D2<25,"Healthy",D2<30,"Overweight",TRUE,"Obese") | Excel 2019 or 365. Use nested IF in older versions |
| Lowest healthy weight | =ROUND(18.5*(B2/100)^2,1) | Reverse BMI at 18.5 |
| Highest healthy weight | =ROUND(24.9*(B2/100)^2,1) | Reverse BMI at 24.9 |
| Feet and inches to inches | =E2*12+F2 | 5 ft 9 in becomes 69 |
| Child percentile | =NORM.S.DIST(((C2/M)^L-1)/(L*S),TRUE)*100 | L, M and S come from the CDC table |
8.2 Step by step
1 Put weight and height in two columns
Type a header in row 1 and your numbers in row 2. Keep height in centimetres, because that is what most scales and clinic forms use.
Weight 70 kg in A2 and height 175 cm in B2.
2 Calculate BMI
Divide height by 100 to turn centimetres into metres, square it, and divide weight by the result. The brackets matter.
=A2/(B2/100)^2The raw BMI is 22.857. Fill the formula down to do a whole list at once.
3 Round, then add the category
Round to one decimal first, then test the rounded value. That is how this calculator works, and it stops a raw 24.96 from showing as 25.0 but being labelled healthy.
=ROUND(C2,1) =IFS(D2<18.5,"Underweight",D2<25,"Healthy",D2<30,"Overweight",TRUE,"Obese")Rounded BMI 22.9, category Healthy.
4 Add the healthy weight range
This is the reverse BMI formula: multiply the target BMI by height in metres squared.
=ROUND(18.5*(B2/100)^2,1) =ROUND(24.9*(B2/100)^2,1)For 175 cm, a healthy weight runs from 56.7 to 76.3 kg.
5 Child BMI percentile from the CDC table
Download the CDC BMI-for-age LMS file, look up L, M and S for the child's sex and age in months, and plug them in. For a 10-year-old boy (age 120.5 months) L is −2.76565, M is 16.64614 and S is 0.120112.
=NORM.S.DIST(((C2/E2)^D2-1)/(D2*F2),TRUE)*100A boy of 140 cm and 40 kg has BMI 20.41, which is the 90.3rd percentile: overweight for his age.
IFS and NORM.S.DIST are both supported.8.3 The awkward cases
- Height in metres already. If your column holds 1.75, use
=A2/B2^2. Dividing 1.75 by 100 again gives a BMI of 228,571. - Feet and inches in one cell. "5'9" is text. Split it into two number columns first, then use
=E2*12+F2. - Asian cut-offs. Swap the thresholds:
=IFS(D2<18.5,"Underweight",D2<23,"Healthy",D2<27.5,"Overweight",TRUE,"Obese"). - Amputation. Adjust weight first:
=A2/(1-G2/100), where G2 is the total percent missing, for example 5.9.
#DIV/0!. Wrap the formula: =IF(B2="","",A2/(B2/100)^2).8.4 Why your Excel answer might differ from R, Python or this calculator
The metric formula gives identical results everywhere: 22.857 in Excel, R, Python and this page. Small differences come from two places. First, the US formula uses 703 as a shortcut. The exact factor is 703.0696, so 154.3 lb at 68.9 in gives 22.855 in Excel's =703*A2/B2^2 but 22.857 here, because this tool converts to kilograms and metres first. Second, rounding. Excel's ROUND rounds halves away from zero, while some languages round halves to the nearest even digit, and binary floating point can store 22.85 as 22.8499999. These only matter when BMI lands exactly on a half, and they never shift the answer by more than 0.1.
8.5 Charting BMI in Excel
Select your names and rounded BMI values, then choose Insert, then Clustered Column. Add horizontal reference lines at 18.5, 25 and 30 by adding three extra series with the same value in every row and changing their chart type to Line. Colour the healthy band green so the eye goes straight to it.
8.6 Error messages and fixes
| You see | Cause | Fix |
|---|---|---|
#DIV/0! | Height cell empty or zero | Enter height, or wrap in IF(B2="","",...) |
#VALUE! | A number stored as text, often "175 cm" | Type just 175, or use =VALUE(SUBSTITUTE(B2," cm","")) |
#NAME? | IFS in Excel 2016 or older | Use nested IF instead |
#NUM! | Negative base raised to a fractional power in the percentile formula | Check L, M and S were copied from the right row |
##### | Column too narrow | Double-click the column edge to widen it |
📊 9. How to Calculate BMI in R
Rscript, with base R only. Change only the data frame at the top to use your own numbers.9.1 The complete script
R, base only, saves bmi_plot.png at 300 dpi
# ============================================================
# BMI calculator in R: BMI, category, healthy range, child percentile
# StatsUnlock - https://statsunlock.com/bmi-calculator/
# Base R only. No packages to install.
# ============================================================
# ---- 1. Your data (weight in kg, height in cm) ----------------
people <- data.frame(
name = c("A", "B", "C", "D", "E"),
weight = c(70, 102, 48, 76.6, 95),
height = c(175, 170, 172, 175, 183)
)
# Or load a file: people <- read.csv("people.csv")
people <- people[complete.cases(people), ] # drop rows with missing values
# ---- 2. BMI and category --------------------------------------
h_m <- people$height / 100
people$bmi <- people$weight / h_m^2
people$bmi1 <- round(people$bmi, 1)
people$category <- cut(people$bmi1,
breaks = c(-Inf, 18.5, 25, 30, Inf),
right = FALSE,
labels = c("Underweight", "Healthy", "Overweight", "Obese"))
# ---- 3. Healthy weight range for each height -------------------
people$healthy_min <- round(18.5 * h_m^2, 1)
people$healthy_max <- round(24.9 * h_m^2, 1)
print(people[, c("name", "bmi1", "category", "healthy_min", "healthy_max")])
# ---- 4. Child BMI-for-age percentile (CDC LMS method) ---------
# Boy, 10 years 0 months (use the 120.5 month row of bmiagerev.csv)
L <- -2.76565; M <- 16.64614; S <- 0.120112
child_bmi <- 40 / 1.40^2
z <- ((child_bmi / M)^L - 1) / (L * S)
pct <- pnorm(z) * 100
cat(sprintf("Child BMI %.2f is at percentile %.1f\n", child_bmi, pct))
# ---- 5. Plain-English verdict ---------------------------------
for (i in seq_len(nrow(people))) {
cat(sprintf("%s: BMI %.1f, %s. Healthy weight %.1f to %.1f kg.\n",
people$name[i], people$bmi1[i], as.character(people$category[i]),
people$healthy_min[i], people$healthy_max[i]))
}
# ---- 6. One 300 dpi figure ------------------------------------
cols <- c(Underweight = "#93c5fd", Healthy = "#16a34a",
Overweight = "#f59e0b", Obese = "#ea580c")
png("bmi_plot.png", width = 2000, height = 1400, res = 300)
par(mar = c(4, 4, 2, 1))
bp <- barplot(people$bmi1, names.arg = people$name,
col = cols[as.character(people$category)],
ylim = c(0, 40), ylab = "BMI (kg/m2)",
main = "BMI with WHO cut-offs")
abline(h = c(18.5, 25, 30), lty = 2, col = "grey40")
text(bp, people$bmi1 + 1.5, people$bmi1, cex = 0.8)
dev.off()
cat("Saved bmi_plot.png\n")
Expected output: A 22.9 Healthy, B 35.3 Obese, C 16.2 Underweight, D 25.0 Overweight, E 28.4 Overweight, and the child at the 90.3rd percentile. These match the calculator and the Excel steps exactly.
9.2 What each line does
| Line or function | What it does, and why it is there |
|---|---|
| data.frame(...) | Holds one row per person. Swap in read.csv() to load a real file. |
| complete.cases() | Drops rows with a missing weight or height, so no BMI comes out as NA. |
| height / 100 | Turns centimetres into metres. Forgetting it gives a BMI 10,000 times too small. |
| round(bmi, 1) | Rounds before classifying, the same rule as the calculator. |
| cut(..., right = FALSE) | Makes each band include its lower edge, so 25.0 is overweight and 18.5 is healthy. |
| pnorm(z) | Turns the CDC z-score into a percentile. |
| png(..., res = 300) | Saves a print-quality figure, 2000 by 1400 pixels at 300 dpi. |
9.3 What the figure shows
One bar per person, coloured by category, with dashed lines at 18.5, 25 and 30. Person D sits right on the 25 line, which is the borderline case worth a second look with a tape measure.
9.4 Problems and fixes
- Every BMI is under 1. Height was already in metres. Remove the
/ 100. - Category is NA. A BMI of exactly 18.5 with
right = TRUEfalls outside the band. Keepright = FALSE. - Error in png(). The folder is read-only. Run
setwd("~")first.
9.5 Handy one-liners
bmi <- function(kg, cm) kg / (cm / 100)^2 # metric
bmi_us <- function(lb, inch) 703.0696 * lb / inch^2 # US units, exact factor
weight_for_bmi <- function(target, cm) target * (cm / 100)^2 # reverse BMI
adj_weight <- function(kg, pct_missing) kg / (1 - pct_missing / 100) # amputee🐍 10. How to Calculate BMI in Python
Install with pip install numpy matplotlib if you do not already have them.
10.1 The complete script
Python 3, saves bmi_plot.png at 300 dpi
# ============================================================
# BMI calculator in Python: BMI, category, healthy range, child percentile
# StatsUnlock - https://statsunlock.com/bmi-calculator/
# ============================================================
from decimal import Decimal, ROUND_HALF_UP
from statistics import NormalDist
import numpy as np
import matplotlib.pyplot as plt
# ---- 1. Your data (weight in kg, height in cm) ----------------
names = ["A", "B", "C", "D", "E"]
weight = np.array([70, 102, 48, 76.6, 95], dtype=float)
height = np.array([175, 170, 172, 175, 183], dtype=float)
# Or load a file:
# import pandas as pd; df = pd.read_csv("people.csv").dropna()
def round1(x):
"""Round half away from zero, the same as Excel and this calculator."""
return float(Decimal(str(x)).quantize(Decimal("0.1"), rounding=ROUND_HALF_UP))
def category(b):
if b < 18.5: return "Underweight"
if b < 25: return "Healthy"
if b < 30: return "Overweight"
return "Obese"
# ---- 2. BMI, category and healthy range -----------------------
h_m = height / 100
bmi = weight / h_m**2
bmi1 = [round1(b) for b in bmi]
cats = [category(b) for b in bmi1]
lo = 18.5 * h_m**2
hi = 24.9 * h_m**2
for n, b, c, a, z in zip(names, bmi1, cats, lo, hi):
print(f"{n}: BMI {b:.1f}, {c}. Healthy weight {a:.1f} to {z:.1f} kg.")
# ---- 3. Child BMI-for-age percentile (CDC LMS method) ---------
L, M, S = -2.76565, 16.64614, 0.120112 # boy, 120.5 months
child_bmi = 40 / 1.40**2
z = ((child_bmi / M)**L - 1) / (L * S)
print(f"Child BMI {child_bmi:.2f} is at percentile {NormalDist().cdf(z)*100:.1f}")
# ---- 4. One 300 dpi figure ------------------------------------
colours = {"Underweight": "#93c5fd", "Healthy": "#16a34a",
"Overweight": "#f59e0b", "Obese": "#ea580c"}
fig, ax = plt.subplots(figsize=(6.7, 4.7))
bars = ax.bar(names, bmi1, color=[colours[c] for c in cats])
for y in (18.5, 25, 30):
ax.axhline(y, ls="--", color="grey", lw=1)
ax.bar_label(bars, labels=[f"{b:.1f}" for b in bmi1], padding=3)
ax.set_ylim(0, 40)
ax.set_ylabel("BMI (kg/m²)")
ax.set_title("BMI with WHO cut-offs")
fig.tight_layout()
fig.savefig("bmi_plot.png", dpi=300)
print("Saved bmi_plot.png")
Expected output matches R and Excel: A 22.9, B 35.3, C 16.2, D 25.0, E 28.4, child at the 90.3rd percentile.
10.2 What each line does
| Line or function | What it does, and why it is there |
|---|---|
| np.array(..., dtype=float) | Stores the numbers so the whole list is calculated in one line. |
| round1() | Python's built-in round() rounds halves to even. This helper rounds halves up, like Excel. |
| category() | Applies the WHO bands to the rounded BMI. Change 25 and 30 to 23 and 27.5 for Asian cut-offs. |
| NormalDist().cdf(z) | Turns the CDC z-score into a percentile without needing SciPy. |
| fig.savefig(dpi=300) | Saves a figure sharp enough for a report or poster. |
10.3 What the figure shows
The same five bars as the R version, coloured by category with dashed cut-off lines. The two scripts and Excel agree to one decimal place.
10.4 Problems and fixes
- ModuleNotFoundError: numpy. Run
pip install numpy matplotlib. - round(22.85, 1) gives 22.8. That is banker's rounding plus floating point. Use the
round1()helper. - ZeroDivisionError. A height of 0 slipped in. Filter with
height > 0first.
10.5 Handy one-liners
bmi = lambda kg, cm: kg / (cm / 100) ** 2
bmi_us = lambda lb, inch: 703.0696 * lb / inch ** 2
weight_for_bmi = lambda target, cm: target * (cm / 100) ** 2
adj_weight = lambda kg, pct: kg / (1 - pct / 100)👥 11. What Is a Healthy BMI for Women and Men by Age?
For adults aged 20 and over, a healthy BMI is 18.5 to 24.9 for both women and men at every age. The cut-offs do not change with sex or age. What does change is how much of that weight is fat. At the same BMI, women usually carry about 10 percentage points more body fat than men, and body fat creeps up with age as muscle is lost.
11.1 Estimated body fat at the same BMI, by age and sex
| Age | Men: body fat at BMI | Women: body fat at BMI | ||||
|---|---|---|---|---|---|---|
| 22 | 25 | 30 | 22 | 25 | 30 | |
| 20 years | 15% | 18% | 24% | 26% | 29% | 35% |
| 40 years | 19% | 23% | 29% | 30% | 34% | 40% |
| 60 years | 24% | 28% | 34% | 35% | 38% | 44% |
| 80 years | 29% | 32% | 38% | 39% | 43% | 49% |
Values from the Deurenberg et al. (1991) equation used in this BMI calculator, accurate to about ±4 points for one person. What to conclude: a 20-year-old woman with a BMI of 22 (about 26% fat) carries about the same body fat as a 20-year-old man with a BMI of 30 (about 24%), and every extra 10 years of age adds about 2 to 3 points at the same BMI.
11.2 Which BMI rules apply at each age
| Age group | How BMI is judged | Healthy range | Use this tab |
|---|---|---|---|
| Under 2 years | BMI is not used; WHO weight-for-length charts instead | Not applicable | None (ask your health visitor) |
| 2 to 19 years | CDC percentile for age and sex | 5th to under 85th percentile | Child & Teen |
| 20 to 64 years | Adult cut-offs, same for women and men | 18.5 to 24.9 (Asian 18.5 to 22.9) | Adult BMI |
| 65 years and over | Adult cut-offs, read with care | 18.5 to 24.9 officially; mortality is lowest at a higher BMI in many older adults | Adult BMI |
11.3 Is a slightly higher BMI better after 65?
Often, yes. A meta-analysis of older adults by Winter et al. (2014) found that the risk of death rose below a BMI of about 23 and was lowest around 27 to 28. Extra reserve helps older people survive illness and falls, while losing weight late in life often means losing muscle. If you are over 65 with a BMI between 25 and 30, talk to your doctor before trying to lose weight.
📏 12. BMI and Waist Size: What Is Your Disease Risk?
BMI tells you how heavy you are for your height. Your waist tells you where the weight sits. Fat around the middle is linked more closely with type 2 diabetes and heart disease, so doctors combine the two numbers. Measure your waist with a tape measure at the level of your navel, then find your row below.
12.1 Disease risk by BMI category and waist size
| BMI category | BMI | Waist: men up to 102 cm (40 in), women up to 88 cm (35 in) | Waist: men over 102 cm, women over 88 cm |
|---|---|---|---|
| Underweight | Below 18.5 | Not raised by weight | Not raised by weight |
| Healthy weight | 18.5 to 24.9 | Not raised by weight | Increased |
| Overweight | 25.0 to 29.9 | Increased | High |
| Obesity class I | 30.0 to 34.9 | High | Very high |
| Obesity class II | 35.0 to 39.9 | Very high | Very high |
| Obesity class III | 40.0 and above | Extremely high | Extremely high |
Risk of type 2 diabetes, high blood pressure and heart disease relative to a healthy weight and waist. Based on the US National Heart, Lung, and Blood Institute (NHLBI) adult obesity guidelines; see the NHLBI waist and BMI guidance, as of 2026. Run the Adult BMI calculator with your waist entered to highlight your row.
12.2 Waist cut-offs for Asian adults and waist-to-height ratio
| Measure | Healthy | Increased risk | High risk |
|---|---|---|---|
| Waist, Asian men | Under 90 cm (35.4 in) | 90 cm or more | |
| Waist, Asian women | Under 80 cm (31.5 in) | 80 cm or more | |
| Waist-to-height ratio, everyone | 0.40 to 0.49 | 0.50 to 0.59 | 0.60 or more |
What to conclude: if your BMI is healthy but your waist is over half your height, your risk is still raised. A falling waist is a better sign of progress than a falling BMI, especially when you are building muscle.
🤰 13. BMI and Pregnancy: How Much Weight Should You Gain?
BMI during pregnancy is not meaningful, because the baby, placenta and extra fluid all add weight. Instead, doctors use your BMI from before pregnancy to set a healthy weight-gain range. Enter your pre-pregnancy weight in the Adult BMI calculator, then read your row below.
| Pre-pregnancy BMI | Category | Total gain, one baby | Total gain, twins |
|---|---|---|---|
| Below 18.5 | Underweight | 28 to 40 lb (12.7 to 18.1 kg) | 50 to 62 lb (22.7 to 28.1 kg) |
| 18.5 to 24.9 | Healthy weight | 25 to 35 lb (11.3 to 15.9 kg) | 37 to 54 lb (16.8 to 24.5 kg) |
| 25.0 to 29.9 | Overweight | 15 to 25 lb (6.8 to 11.3 kg) | 31 to 50 lb (14.1 to 22.7 kg) |
| 30.0 and above | Obesity | 11 to 20 lb (5.0 to 9.1 kg) | 25 to 42 lb (11.3 to 19.1 kg) |
Source: Institute of Medicine (2009) recommendations, as published by the CDC weight gain during pregnancy page, as of 2026. The CDC notes its underweight twin range differs from the original IOM guidance. For triplets or more, ask your midwife or doctor. What to conclude: the higher your starting BMI, the less weight you need to gain, but no one should try to lose weight during pregnancy without medical advice.
🌎 14. What Is a Typical Body Mass Index Around the World?
The typical adult BMI worldwide now sits just under the overweight line. A pooled analysis of 19.2 million adults in 200 countries (NCD Risk Factor Collaboration, 2016) found the global average rose steadily over four decades:
| Year | Average BMI, men | Average BMI, women | What it means |
|---|---|---|---|
| 1975 | 21.7 | 22.1 | Middle of the healthy range |
| 2014 | 24.2 | 24.4 | Close to the top of the healthy range |
| Change | +2.5 | +2.3 | About 7 kg more for a 170 cm adult |
These are age-standardised global averages. Many countries sit well above or below them. What to conclude: a "typical" BMI is not the same as a healthy one. If your BMI matches the average, you may already be near the 25 line.
14.1 How your result compares
___ Run the Adult BMI calculator above to compare your BMI with the global average for your sex.
🔬 15. How This BMI Calculator Was Built and Tested
The StatsUnlock Team built this BMI calculator from primary sources, not from other calculators, and checked every mode against independent code before publishing.
| Part | Method and source | How we checked it |
|---|---|---|
| Adult BMI | Weight ÷ height²; WHO, WHO Asian (2004), JASSO (2002) and NICE cut-offs | 9 adult samples matched an independent Python script to 2 decimals |
| Units | Height in cm, m, ft + in or in; weight in kg, lb or st + lb. Exact factors (1 lb = 0.45359237 kg, 1 st = 14 lb = 6.35029318 kg, 1 in = 2.54 cm), not the 703 shortcut | All 12 height and weight unit pairs give the same BMI; switching units converts values without loss |
| Rounding | Round once to 1 decimal, half away from zero; classify the rounded value | 24.96 shows 25.0 and is labelled overweight |
| Child percentile | CDC 2000 BMI-for-age LMS table, all 432 monthly rows for boys and girls, half-month row for each completed month | 6 child samples matched independent code; 10-year-old boy median 16.6 matches CDC |
| Amputee BMI | Osterkamp (1995) segment percentages | 5 limb combinations checked by hand |
| Body fat, BMR, calories | Deurenberg (1991), Mifflin-St Jeor (1990); safety floors of 1,500 and 1,200 kcal | No deficit plan shown below BMI 18.5 or for flagged athletes |
| Input checks | Range limits, metre-entry detection, decimal comma, implausible BMI guard | 9 error cases tested; no blank or NaN result possible |
| Devices | Single HTML file, no tracking, nothing calculated until you press Calculate | Tested at 320, 375, 414 and 1,100 px wide |
Your numbers never leave your browser. The calculator runs entirely on your device and stores nothing. Last reviewed: . Found an error? Tell us through the contact page.
🎯 16. Example BMI Calculator Results (8 Worked Cases)
Eight real-world cases run through the BMI calculator, from a textbook adult to an amputee and a reverse BMI goal. Each shows the numbers, what they mean and what to do next.
Adult man, 70 kg and 175 cm
The clean case where BMI does its job well.
A 30-year-old man with a typical build weighs 70 kg and is 175 cm tall.
| Measure | Value | Note |
|---|---|---|
| BMI (exact) | 22.86 | 70 ÷ 1.75² |
| BMI (rounded) | 22.9 | Used for the category |
| Category | Healthy weight | WHO 18.5 to 24.9 |
| Healthy range | 56.7 to 76.3 kg | 18.5 and 24.9 × 3.0625 |
| BMI Prime | 0.91 | Under 1.00 |
| Body fat (Deurenberg) | 18.1% | Estimate ±4 points |
What it means: This man is 13.3 kg above the bottom of his healthy range and 6.3 kg below the top. There is no weight-related reason to change anything.
How to use it: Record it as a baseline and re-check every few months. Add a waist measurement for a fuller picture.
Obesity class II, 102 kg and 170 cm
A result that warrants a conversation with a doctor.
A 45-year-old man, lightly active, weighs 102 kg at 170 cm.
| Measure | Value | Note |
|---|---|---|
| BMI (exact) | 35.29 | 102 ÷ 1.70² |
| Category | Obesity class II | WHO 35.0 to 39.9 |
| Healthy range | 53.5 to 72.0 kg | At 170 cm |
| Above range by | 30.0 kg | 102 − 72.0 |
| First 5% goal | 5.1 kg | Meaningful health benefit |
| Body fat (Deurenberg) | 36.5% | Estimate |
What it means: A BMI of 35.3 is linked with higher risk of diabetes, high blood pressure and sleep apnoea. It also meets the BMI threshold for a bariatric surgery assessment.
How to use it: Start with a first goal of 5% (about 5 kg). Ask a GP about blood pressure, HbA1c and support programmes.
Underweight, 48 kg and 172 cm
Where the right next step is a check-up, not a diet.
A 22-year-old woman weighs 48 kg at 172 cm.
| Measure | Value | Note |
|---|---|---|
| BMI (exact) | 16.22 | 48 ÷ 1.72² |
| Category | Underweight (moderate) | WHO grade 2, 16.0 to 16.9 |
| Healthy range | 54.7 to 73.7 kg | At 172 cm |
| Below range by | 6.7 kg | 54.7 − 48 |
| Calorie plan | Not shown | Safety rule below 18.5 |
What it means: A BMI of 16.2 can affect bones, periods, energy and immunity. The calculator shows supportive guidance and a helpline instead of any calorie target.
How to use it: Book a GP appointment, especially if weight loss was unplanned. A dietitian can support healthy weight gain.
Right on the line, 76.6 kg and 175 cm
How rounding decides the label at 25.0.
A 40-year-old man weighs 76.6 kg at 175 cm.
| Measure | Value | Note |
|---|---|---|
| BMI (exact) | 25.01 | 76.6 ÷ 3.0625 |
| BMI (rounded) | 25.0 | Used for the label |
| Category | Overweight | WHO 25.0 to 29.9 |
| Above range by | 0.3 kg | Top of range 76.3 kg |
What it means: Technically overweight, but 0.3 kg is less than a day of normal weight swing. One reading on the line means very little on its own.
How to use it: Measure your waist. Under half your height means low risk. Track the trend over 3 months rather than reacting to one number.
Amputee, below-knee, 70 kg and 175 cm
Why an unadjusted BMI is falsely low.
A 45-year-old man with a left below-knee amputation weighs 70 kg on the scale, without his prosthesis.
| Measure | Value | Note |
|---|---|---|
| Measured weight | 70.0 kg | Without the limb |
| Limb loss | 5.9% | Lower leg and foot (Osterkamp) |
| Adjusted weight | 74.4 kg | 70 ÷ 0.941 |
| Unadjusted BMI | 22.9 | Understates weight for height |
| Adjusted BMI | 24.3 | Used for the category |
What it means: Without adjustment he looks comfortably healthy. Adjusted, he is near the top of the healthy range, which matters when planning weight around a prosthesis.
How to use it: Use the adjusted BMI with your prosthetist and dietitian. A healthy measured weight for him is 53.3 to 71.8 kg.
Athlete, 95 kg and 183 cm
When BMI says overweight but the waist says lean.
A 26-year-old rugby player weighs 95 kg at 183 cm, with a waist of 81 cm.
| Measure | Value | Note |
|---|---|---|
| BMI | 28.4 | 95 ÷ 1.83² |
| Category | Overweight | By BMI alone |
| Waist-to-height | 0.44 | 81 ÷ 183, under 0.5 |
| Body fat (formula) | about 24% | Overstated for muscle |
What it means: Muscle is denser than fat, so BMI misreads strong people. His waist-to-height ratio of 0.44 points to low belly fat and low risk.
How to use it: For muscular people, track waist-to-height ratio or body fat by scan, not BMI alone.
Height typed in metres
What the calculator does with a unit mix-up.
Someone types 1.75 into the centimetre box with a weight of 70 kg.
| Measure | Value | Note |
|---|---|---|
| Height entered | 1.75 | In a cm field |
| Naive BMI | 228,571 | Impossible |
| Tool response | Inline message | Under the height box |
| After fix | 22.9 | With 175 cm |
What it means: Unit mix-ups are the most common cause of strange BMI results online. Values outside 50 to 250 cm are rejected, and metre-sized values get a specific hint.
How to use it: Retype 175, or switch units if you think in feet and inches.
Reverse BMI: target weight at 165 cm
Turning a BMI goal into a weight goal.
A 38-year-old South Asian woman, 165 cm tall, wants to know the top of her healthy range under WHO and under Asian cut-offs.
| Measure | Value | Note |
|---|---|---|
| Height | 165 cm | 1.65² = 2.7225 |
| Weight at BMI 24.9 | 67.8 kg | WHO top of healthy |
| Weight at BMI 22.9 | 62.3 kg | Asian top of healthy |
| Weight at BMI 18.5 | 50.4 kg | Bottom of healthy |
| Difference | 5.4 kg | Between the two standards |
What it means: The standard she uses changes her goal by 5.4 kg. The Asian cut-off reflects higher diabetes risk at a lower BMI.
How to use it: Use the Reverse BMI tab with the Asian standard, and set 62.3 kg as the long-term ceiling.
📏 17. How Do You Measure Height and Weight for an Accurate BMI?
A BMI calculator is only as accurate as the numbers you give it. A 2 cm height error shifts BMI by about 0.5 points.
- Weigh in the morning. After using the toilet and before eating or drinking. Your weight can swing by 1 to 2 kg across a day.
- Use the same scale each time. Two home scales can disagree by a kilogram or more.
- Put the scale on a hard floor. Carpet makes many scales read low.
- Wear light clothing, no shoes. Shoes and jeans can add 1.5 kg.
- Measure height against a flat wall. Heels, bottom, shoulders and head touching the wall, feet together.
- Look straight ahead. Rest a hardback book flat on your head, square to the wall, and mark underneath it.
- Measure from floor to mark with a metal tape. Read to the nearest 0.5 cm.
- Measure your waist with a tape measure at the level of your navel, after breathing out gently, without pulling the tape tight.
- For a child, use exact age in months. A child who is 9 years 11 months is not 10.
- For an amputee, weigh without a prosthesis, or subtract its weight. Then use the Amputee tab.
| Input | Unit | Typical adult range | Accepted by this tool |
|---|---|---|---|
| Height | cm, m, ft + in, or in | 145 to 200 cm | 50 to 250 cm |
| Weight | kg, lb, or st + lb | 40 to 150 kg | 10 to 400 kg |
| Age (adult) | years | 18 to 90 | 18 to 100 |
| Age (child) | years + months | 2 to 19 | 2 y 0 m to 19 y 11 m |
| Waist | cm or in | 60 to 120 cm | 40 to 250 cm |
✅ 18. When Should You Use a BMI Calculator?
- You want a quick check of weight for height
- You are tracking change over months
- A form asks for your BMI
- You want a healthy weight range for your height
- A child needs a BMI-for-age percentile
- You are pregnant
- You are a body-builder or elite athlete
- You are over 75 and losing muscle
- The child is under 2 years old
- You need a diagnosis rather than a screen
Four examples
- A man checking a new year goal: use the Adult tab, then the calorie plan.
- A parent after a school weigh-in: use the Child & Teen tab for a CDC percentile.
- A veteran with a below-knee amputation: use the Amputee tab so BMI is not falsely low.
- Someone planning a target weight: use the Reverse BMI tab with target BMI 24.9.
Decision guide
| Your situation | Better tool |
|---|---|
| Muscular build | Waist-to-height ratio or a body fat scan |
| Under 2 years old | WHO weight-for-length charts |
| Pregnant | Pre-pregnancy BMI plus weight-gain guidance from your midwife |
| Need exact body fat | DEXA scan |
🔧 19. Troubleshooting and Common Errors
Most BMI calculator problems come from units, rounding or the wrong standard. Find your symptom below.
Another website gives a different BMI
Cause: most sites use 703 for US units and round at different points. Some show the raw number, some round down. Fix: compare the metric answer. This tool converts pounds and inches to kg and m with exact factors, then rounds once to one decimal, half away from zero. The difference is never more than 0.1.
My category changed but my BMI looks the same
Cause: you switched the cut-off standard. Fix: check the standard box. WHO, Asian, Japanese and NICE labels differ for the same number.
The tool says height is out of range
Cause: you probably typed metres (1.75) into the centimetre box. Fix: type 175, or switch units to feet and inches.
My child's result says use percentiles
Cause: the age is under 18 in the Adult tab. Fix: open the Child & Teen tab and enter years and months.
No calorie plan appears
Cause: the plan only appears when your BMI is above the healthy range. Below 18.5 the tool shows a gain target instead. Fix: nothing to fix. This is a safety rule.
Reverse BMI will not accept my target
Cause: weight targets below 18.5 are underweight. Fix: the tool shows the lowest healthy weight for your height instead.
My amputee BMI is higher than my normal BMI
Cause: that is correct. Your measured weight is missing the limb, so the raw BMI is too low. Fix: use the adjusted value for health decisions.
The Download TXT button is grey
Cause: nothing has been calculated yet, or you changed a sample or unit. Fix: press Calculate BMI again.
Excel shows #DIV/0!
Cause: an empty height cell. Fix: see Section 8.6.
The charts do not appear
Cause: a script blocker stopped the chart library loading. Fix: allow cdnjs.cloudflare.com or reload the page. All numbers and tables still work without charts.
⚠️ 20. What Are the Limitations of a BMI Calculator?
Every BMI calculator rests on the same assumptions. Here is what each one means for you, and what to use when it fails.
- Weight is mostly fat when BMI is high. If you are very muscular, BMI overstates fat. Alternative: waist-to-height ratio or a DEXA scan.
- Your body shape is average for your height. Very tall people tend to score slightly high and very short people slightly low. Alternative: the Trefethen new BMI (1.3 × W ÷ H2.5), shown in your results table.
- Fat distribution does not matter. It does: belly fat carries more risk. BMI can call someone healthy who has a large waist. Alternative: add your waist in the advanced options.
- Ethnic background does not change risk. For many Asian, Black and Middle Eastern adults, risk starts at a lower BMI, so WHO cut-offs understate it. Alternative: choose the Asian or NICE standard.
- Limb segments weigh the population average. Real limbs vary, and partial thigh amputations fall between 5.9% and 16%. The adjusted BMI may be off by up to 1 point.
- The body fat formula fits your group. Deurenberg's equation came from Dutch adults and can be off by about 4 percentage points for one person.
- Calorie maths is linear. The 7,700 kcal per kg rule overstates long-term weight loss, because the body adapts. Expect slower progress after the first few months.
🏁 21. Conclusion
This BMI calculator turns your height and weight into a body mass index of ___, which falls in the ___ category. Run the calculator above to see your own summary here.
BMI is a fast, free and widely used screen. It works best when you treat it as the first question about weight and health, not the final answer. Check your waist, think about your build, and use the right standard for your background.
Whatever your number, small changes held for months beat large changes held for weeks. Re-measure every four weeks under the same conditions and use this BMI calculator to track the trend.
❓ 22. Frequently Asked Questions About BMI
Q1. What is BMI and how is it calculated?
BMI, or body mass index, is your weight in kilograms divided by your height in metres squared. For example, 70 kg at 1.75 m gives 70 divided by 3.0625, which is 22.9. In pounds and inches, multiply weight by 703 and divide by height in inches squared. The result is a single number used to screen adults for underweight, healthy weight, overweight and obesity.
Q2. What is a healthy BMI for an adult?
A healthy adult BMI is 18.5 to 24.9 under the WHO standard. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obese. These cut-offs are the same for men and women. For South Asian, Chinese and other Asian adults, the healthy range ends at 22.9, because health risks start at a lower BMI.
Q3. Is the BMI calculator different for men and women?
No. Adult BMI uses the same formula and the same cut-offs for men and women. What differs is body composition: at the same BMI, women usually carry more body fat than men. That is why this calculator asks for sex. It does not change your BMI, but it changes the body fat and calorie estimates in your results.
Q4. What BMI is considered obese?
An adult BMI of 30 or more is obese under the WHO standard. Obesity is split into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or more). Under Asian and NICE lower thresholds, obesity starts at 27.5. In Japan, a BMI of 25 or more is classed as obesity.
Q5. What is the Asian BMI cut-off?
For Asian adults, the WHO expert consultation recommends treating 23 to 27.4 as overweight and 27.5 or more as obese. The healthy range is 18.5 to 22.9. These lower cut-offs exist because many Asian people carry more body fat and develop type 2 diabetes at a lower BMI than white Europeans. Choose WHO Asian in the standard box to apply them.
Q6. Do Black women need a different BMI calculator?
The formula is the same, but the cut-offs used to judge risk can differ. UK NICE guidance uses lower thresholds for Black African and African-Caribbean adults: 23 for overweight and 27.5 for obesity, based on diabetes risk. Some studies also suggest BMI can overestimate fat in Black women. Choose the NICE option to see both views.
Q7. How do you calculate BMI for an amputee?
Divide your measured weight by one minus the share of body weight that is missing, then calculate BMI with that adjusted weight. A below-knee amputation removes about 5.9% of body weight, so 70 kg becomes 70 divided by 0.941, which is 74.4 kg. At 175 cm, the adjusted BMI is 24.3, not 22.9. The Amputee tab does this automatically.
Q8. What is a reverse BMI calculator?
A reverse BMI calculator works backwards: you enter a target BMI and your height, and it tells you the weight that gives that BMI. The formula is weight equals BMI times height in metres squared. For example, BMI 24.9 at 165 cm is 67.8 kg. It can also find the height that gives a BMI at a known weight.
Q9. How is BMI calculated for children and teens?
Children aged 2 to 19 use the same BMI formula, but the result is compared with other children of the same age and sex on CDC growth charts. Under the 5th percentile is underweight, the 5th to under the 85th is healthy, the 85th to under the 95th is overweight, and the 95th or above is obesity. Use the Child & Teen tab.
Q10. Is BMI accurate for athletes and muscular people?
Not very. BMI cannot tell muscle from fat, so muscular people often score as overweight while carrying little fat. A rugby player at 95 kg and 183 cm has a BMI of 28.4. A waist-to-height ratio under 0.5 or a body fat scan gives a fairer picture. The athlete option in this calculator flags this.
Q11. What does BMI not tell you?
BMI does not measure body fat, muscle, bone density or where fat is stored. Belly fat carries more health risk than fat on the hips, yet two people with the same BMI can differ greatly in waist size. BMI also ignores age, ethnicity and fitness. Use it as a first screen alongside waist size and blood tests.
Q12. How many calories should I eat to reach a healthy BMI?
Start with your maintenance calories, then eat about 500 kcal a day less, which leads to roughly 0.5 kg of loss a week. This calculator estimates maintenance with the Mifflin-St Jeor equation and never goes below 1,500 kcal for men or 1,200 kcal for women. It shows no deficit plan if your BMI is already under 18.5.
Q13. What BMI do you need for bariatric surgery?
Current ASMBS and IFSO guidance recommends assessment for weight-loss surgery at a BMI of 35 or more, whatever your other health conditions. People with type 2 diabetes or other metabolic disease may qualify at 30 to 34.9. For Asian adults, 27.5 or more may be considered. Insurers and health services can apply stricter rules.
Q14. What BMI is considered underweight or anorexic?
An adult BMI under 18.5 is underweight. WHO grades it as mild (17 to 18.4), moderate (16 to 16.9) and severe (below 16). Doctors also use BMI to grade the severity of anorexia, but an eating disorder is diagnosed from thoughts and behaviour, not weight alone. If you are struggling with eating, please speak to a doctor.
Q15. Is the NHS BMI calculator the same as this one?
Yes, the core formula is identical: weight in kg divided by height in metres squared, with the same adult categories. Like NHS guidance, this tool offers lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, and uses percentiles for children instead of adult cut-offs. You can also enter weight in stones and pounds, as on the NHS tool.
Q16. Why is waist-to-height ratio better than BMI?
Waist-to-height ratio measures belly fat, which is more closely linked to heart disease and diabetes than total weight. Divide your waist by your height, using the same units. Under 0.5 is the usual healthy target. A systematic review found it predicted cardiometabolic risk better than BMI. Add your waist with a tape measure in the advanced options.
Q17. Is there a 3D BMI calculator?
Some sites show a 3D body model that changes shape with your BMI. These are illustrations, not measurements. This calculator draws a simple shaded body preview at your BMI for the same purpose. For a real 3D picture of your body, clinics use 3D body scanners or DEXA, which measure fat and muscle directly.
Q18. How do you calculate BMI in Excel?
Put weight in kilograms in A2 and height in centimetres in B2, then type =A2/(B2/100)^2 in C2. For 70 kg and 175 cm that returns 22.857. Use =ROUND(C2,1) to get 22.9, and an IFS formula to add the category. Section 8 of this page walks through every step with screenshots.
Q19. How do you calculate BMI in R?
Store weight in kg and height in cm, then compute weight / (height / 100)^2. Round with round(bmi, 1) and classify with cut() using breaks at 18.5, 25 and 30 and right = FALSE, so 25.0 counts as overweight. The complete base R script in Section 9 also computes a child percentile with pnorm() and saves a 300 dpi chart.
Q20. How do you calculate BMI in Python?
Use bmi = weight / (height / 100) ** 2 with NumPy arrays to do many people at once. Python's round() rounds halves to even, so use a Decimal helper to round like Excel. For child percentiles, statistics.NormalDist().cdf() converts the CDC z-score without SciPy. The full script in Section 10 prints each verdict and saves a 300 dpi figure.
📎 23. Cite This Tool
StatsUnlock Team. (2026). BMI calculator: Adult, child percentile, Asian cut-off and amputee adjusted [Interactive tool]. StatsUnlock. https://statsunlock.com/bmi-calculator/@misc{statsunlock_bmi_2026, author={{StatsUnlock Team}}, title={BMI Calculator: Adult, Child Percentile, Asian Cut-off and Amputee Adjusted}, year={2026}, publisher={StatsUnlock}, note={Interactive tool}, url={https://statsunlock.com/bmi-calculator/}}Body mass index was calculated as weight in kilograms divided by height in metres squared using the StatsUnlock BMI Calculator (2026), classified with WHO adult cut-offs. For participants aged 2 to 19 years, BMI-for-age percentiles were computed with the CDC 2000 LMS reference. For amputees, weight was adjusted using the segment proportions of Osterkamp (1995).🔗 24. Related Tools
More free StatsUnlock calculators to use alongside this BMI calculator, from age and goal dates to measurement.
Find your exact age in years, months and days, the input the child BMI percentile needs.
Set a 90-day check-in date for your next BMI and waist measurement.
Plan a weight goal date by adding the weeks from your calorie plan to today.
Turn a weekly weight change into a yearly total for long-term goals.
Measure floor area in square feet or metres for a home gym or workout space.
Work out angles and slopes, from treadmill incline to ramp gradients.
📖 25. Glossary of Terms
- Adjusted BMI
- BMI calculated from an estimated full-body weight, used for people with amputations.
- Amputation
- Surgical or accidental loss of part or all of a limb.
- Bariatric surgery
- Operations on the stomach or gut that help people with severe obesity lose weight.
- BMI (body mass index)
- Weight in kilograms divided by height in metres squared.
- BMI calculator
- A tool that divides weight by height squared and labels the result with a category.
- BMI Prime
- Your BMI divided by 25. A value above 1.00 means above the overweight line.
- BMR (basal metabolic rate)
- Calories your body burns at complete rest in a day.
- Calorie deficit
- Eating fewer calories than you burn, which leads to weight loss over time.
- CDC
- The US Centers for Disease Control and Prevention, which publishes the child BMI growth charts.
- Cut-off
- The BMI value where one category ends and the next begins.
- DEXA scan
- A low-dose X-ray that measures fat, muscle and bone separately.
- Healthy weight range
- The weights that give a BMI of 18.5 to 24.9 at your height.
- LMS method
- A way to turn a child's BMI into a percentile using three numbers: L (skew), M (median) and S (spread).
- NICE
- The UK National Institute for Health and Care Excellence, which writes NHS clinical guidance.
- Obesity
- An adult BMI of 30 or more (27.5 or more under Asian cut-offs).
- Overweight
- An adult BMI of 25 to 29.9 (23 to 27.4 under Asian cut-offs).
- Percentile
- The percent of children of the same age and sex with a lower BMI.
- Ponderal index
- Weight divided by height cubed. It changes less with height than BMI.
- Reverse BMI
- Working backwards from a BMI to the weight or height that gives it.
- TDEE
- Total daily energy expenditure: BMR multiplied by an activity factor.
- Underweight
- An adult BMI below 18.5.
- Waist-to-height ratio
- Waist divided by height. Under 0.5 is the usual healthy target.
- Z-score
- How many standard deviations a value sits above or below the average.
📚 26. References
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- Cole, T. J., & Green, P. J. (1992). Smoothing reference centile curves: The LMS method and penalized likelihood. Statistics in Medicine, 11(10), 1305–1319. https://doi.org/10.1002/sim.4780111005
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