About this tool
This free BMI calculator works out your Body Mass Index from your height and weight — in metric (cm / kg) or imperial (ft-in / lb) — and shows which category the result falls into.
It runs in your browser; nothing is uploaded and no measurement is stored.
BMI is a rough screening figure, not a diagnosis. It was never designed to assess an individual, and it cannot tell muscle from fat. Please read the limitations below before drawing any conclusion from your number — and speak to a doctor rather than to a calculator about your health.
How BMI categories work
For adults, the World Health Organization uses these ranges:
- Under 18.5 — Underweight
- 18.5 – 24.9 — Normal weight
- 25 – 29.9 — Overweight
- 30 and over — Obese
The obese range is often split further: 30–34.9 as class I, 35–39.9 as class II, and 40 and above as class III.
These are population categories, not personal verdicts. Someone at 24.9 and someone at 25.0 are not meaningfully different; the boundary is a round number chosen for convenience, not a cliff.
The formula, and where it came from
The arithmetic is simple. In metric it is weight in kilograms divided by height in metres squared. Someone 1.75 m and 70 kg: 70 ÷ (1.75 × 1.75) = 22.9.
In imperial it is weight in pounds divided by height in inches squared, multiplied by 703 — the 703 being purely a unit conversion.
The history explains a lot about its limits. The formula was devised in the 1830s by Adolphe Quetelet, a Belgian mathematician and astronomer. He was not a doctor and had no interest in individual health. He was studying *l'homme moyen* — the average man — and looking for a way to describe the distribution of body sizes across a population.
It became a health measure in 1972, when the physiologist Ancel Keys tested several height-weight formulas and found Quetelet's was the best available proxy for body fat in large studies. He named it the Body Mass Index — and explicitly said it was appropriate for population studies rather than individual diagnosis.
That caveat has been quietly dropped over fifty years of use. The formula was fit for its original purpose and has been doing a different job ever since.
What BMI cannot see
Understanding these makes the number far more useful, because you know when to discount it.
It cannot distinguish muscle from fat. Muscle is denser. A rugby player, a sprinter or anyone who lifts seriously can land in the "obese" range with very low body fat. This is the best-known failure and it is entirely real.
It ignores where fat is carried. Fat around the abdomen carries considerably more health risk than fat on the hips and thighs, and BMI treats the two identically. Waist circumference and waist-to-height ratio capture this better — a waist under half your height is a common rule of thumb.
It does not adjust for age. Older adults lose muscle and bone density, so the same BMI represents more body fat at 70 than at 30. Some evidence suggests a slightly higher BMI is protective in older age.
Thresholds differ by ethnicity. Health risk begins at a lower BMI for people of South Asian, Chinese and other Asian descent, and several health bodies use a lower overweight threshold — around 23 — for these groups. The standard ranges were derived largely from European populations.
It does not apply to children. Children are assessed against age-and-sex percentile charts, not adult categories.
It is not valid in pregnancy, and it is unreliable for anyone with a very short or very tall stature, since the formula squares height while bodies do not scale that way.
What the number is genuinely good for
Despite everything above, BMI survives because it is free, instant and needs only a tape measure and scales — and across large groups it correlates well enough with health outcomes to be useful.
For you personally, the sensible uses are:
As a starting point for a conversation, not the conclusion of one. A result well outside the normal range is worth mentioning to a doctor.
As a trend over time. Your own BMI tracked over months tells you more than a single reading compared against a population range, because it controls for your build.
As one number among several. Waist measurement, blood pressure, activity level, how you feel and how you sleep all matter. BMI on its own answers very little.
How to use it
- Pick your units — metric or imperial
- Enter your height and weight
- Read your result — your BMI value and category update instantly
Good to know
- Measure height properly — without shoes, standing straight. A couple of centimetres shifts the result more than people expect, because height is squared.
- Weigh consistently — same time of day, ideally morning, similar clothing. Body weight swings by one to two kilograms across a day from food and fluid alone.
- Do not chase decimals. BMI 22.4 and 22.9 are the same thing in practice.
- Athletic build? Expect BMI to overstate your result, and use body composition or waist measurement instead.
- Asian ancestry? Several health bodies apply a lower overweight threshold, around 23.
- This is not medical advice. A calculator cannot examine you. Any real concern belongs with a doctor.
Common questions
What is a healthy BMI?
For adults, the World Health Organization treats 18.5 to 24.9 as the normal range, with under 18.5 underweight, 25 to 29.9 overweight and 30 and above obese. These are population categories rather than individual targets, and the boundaries are round numbers chosen for convenience — 24.9 and 25.0 are not meaningfully different.
How is BMI calculated?
In metric, divide your weight in kilograms by your height in metres squared: 70 kg at 1.75 m gives 70 ÷ 3.0625 = 22.9. In imperial, divide pounds by height in inches squared and multiply by 703. The calculator above handles either. Note that height is squared, so a small error in measuring it moves the result noticeably.
Is BMI accurate for everyone?
No, and it was never meant to be. It cannot tell muscle from fat, so athletic people often score as overweight or obese despite low body fat. It ignores where fat is carried, does not adjust for age, and its thresholds were derived largely from European populations — risk begins at a lower BMI for people of South Asian and East Asian descent. It does not apply to children or during pregnancy.
Why does BMI say I am overweight when I am muscular?
Because the formula uses only height and weight, and muscle is denser than fat. Someone who trains seriously carries more weight for their height without carrying more fat, and BMI reads that as excess. This is the best-documented limitation of the measure. Body fat percentage or a waist measurement will describe you far better.
Is BMI different for men and women?
The formula and the adult categories are the same. Women typically carry a higher proportion of body fat at the same BMI, so the number represents something slightly different — one of several reasons BMI is better at describing groups than individuals. Children are assessed on age-and-sex percentile charts instead.
What should I measure instead of, or alongside, BMI?
Waist circumference is the most useful single addition, because fat carried around the abdomen carries more risk than fat elsewhere and BMI cannot see the difference. A waist under half your height is a widely used rule of thumb. Body fat percentage, blood pressure and activity level all add information BMI does not contain.
Is my height and weight sent anywhere?
No. The calculation happens entirely in your browser as you type, nothing is transmitted, and no measurement is stored or logged. The tool works with your connection switched off.
Related tools
- TDEE Calculator — estimate the calories your body uses in a day
- Age Calculator — work out an exact age from a date of birth
- Unit Converter — convert between kilograms, pounds, centimetres and feet
- Water Reminder — a nudge to drink water through the day
