BMI Calculator
Enter your height and weight to get your body mass index, plotted on the full WHO scale rather than dropped into a box. The healthy weight range for your height is shown alongside it.
A healthy BMI for this height is roughly 56.7–76.3 kg.
70 kg ÷ (1.75 m)² = 22.9This is a screening indicator, not a diagnosis. BMI compares weight to height and nothing else. It cannot distinguish muscle from fat, so it misclassifies athletes routinely, and the thresholds were derived largely from European populations — several health bodies use lower cut-offs for people of South Asian descent. It is not designed for children, pregnant people, or adults over about 65. Speak to a doctor before acting on it.
Calculated on your device. Your height and weight are never sent anywhere.
Related tools
How to use the BMI calculator
- Choose your units — metric for kilograms and centimetres, imperial for pounds, feet and inches.
- Enter your height and weight. The result updates as you type; there is no button to press.
- Read the number against the scale. The marker shows where you sit across the whole range rather than only which category you land in.
- Check the healthy weight range underneath, which converts the 18.5 to 24.9 band back into weight for your particular height.
Nothing is uploaded. Both measurements stay in your browser.
What body mass index actually measures
BMI is weight divided by height squared. That is the entire calculation. There is nothing else in it — no age, no sex, no muscle, no bone density, no measure of where on your body the weight sits.
Understanding that one sentence explains almost every argument about BMI. The number is not a measure of fat, or of fitness, or of health. It is a measure of how heavy you are relative to how tall you are, and it is being used as a rough proxy for body fatness because at the scale of whole populations that proxy holds up reasonably well.
The index dates to the 1830s, when the Belgian statistician Adolphe Quetelet was looking for a way to describe the average build of a population. He was doing social statistics, not medicine, and he said explicitly that his index was not meant for individuals. It picked up its modern name in 1972, when the physiologist Ancel Keys compared several weight-for-height formulas and found Quetelet's the best of a limited set for population studies.
So BMI arrived in medicine as the least bad option among cheap measures, and its enduring advantage is exactly that cheapness. Two measurements, no equipment beyond a scale and a tape, no laboratory, no specialist. For tracking obesity across a country, that combination is hard to beat.
The WHO categories, and what they are for
The World Health Organization classification is the one almost every calculator uses:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese class I |
| 35.0 – 39.9 | Obese class II |
| 40.0 and above | Obese class III |
These bands come from large studies linking BMI to death rates and disease incidence. Risk of several conditions — type 2 diabetes, cardiovascular disease, some cancers, joint problems — rises as BMI climbs above the middle band, and rises again in the other direction below it.
What the table hides is that risk is a smooth curve, not a staircase. There is no biological event at 25.0. The boundaries exist because public health needs categories to count and compare, and a continuous variable has to be cut somewhere. Someone whose BMI moves from 24.8 to 25.2 over a winter has not crossed into a different health state; they have crossed a line a committee drew.
This is why the tool plots your value on a bar rather than only naming a band. Where you sit inside a category matters more than which category it is.
Where BMI is wrong, and for whom
The caveats are not footnotes. For a substantial minority of people, BMI gives an actively misleading answer.
Muscular people. Muscle weighs more than fat for the same volume. An athlete in serious training can carry very little body fat and still produce a BMI in the overweight or obese range. The number is not measuring what it appears to measure in that case.
Different ethnic backgrounds. The thresholds came mostly from European population data. People of South Asian, Chinese and several other Asian backgrounds tend to develop metabolic disease at lower BMI values, partly because of where fat is stored rather than how much there is. The WHO and NICE recommend lower action points — roughly 23 and 27.5 — for these groups, and applying the standard table to them understates risk.
Older adults. People lose height with age through spinal compression, and lose muscle mass. Both distort the ratio: shrinking height pushes BMI up even with no weight change, while muscle loss means the same BMI represents proportionally more fat than it did decades earlier.
Pregnancy. BMI is not meaningful during pregnancy. Weight gain is expected and monitored differently.
Children and teenagers. Growth and puberty change body composition constantly. Paediatric assessment uses age-and-sex percentile charts instead.
Very short and very tall people. Because height is squared while bodies do not scale as neat cubes or squares, the index has a mild bias — it tends to read slightly high for tall people and slightly low for short people.
What to measure instead, or as well
If you want a fuller picture without a clinic appointment, waist circumference is the single most useful addition. Fat stored around the abdomen sits alongside internal organs and is far more strongly associated with metabolic risk than fat carried on the hips or thighs. Two people with identical BMI and very different waists have meaningfully different risk profiles.
The simplest form of this is the waist-to-height ratio: keep your waist measurement below half your height. It needs no chart, works across most heights, and holds up better across ethnic groups than BMI does. UK clinical guidance now recommends using it alongside BMI for exactly that reason.
Beyond body measurements, the things that actually describe cardiovascular and metabolic health are blood pressure, resting heart rate, fasting blood glucose, HbA1c and a lipid panel. None of them can be worked out from your weight, and all of them matter more.
Limits and requirements
Adults only. Not valid for children, teenagers, or during pregnancy.
A screening indicator, not a diagnosis. It cannot diagnose anything, and no clinical decision should rest on it alone.
Implausible values are rejected. Entering metres where centimetres are expected produces an error rather than a nonsense figure.
Rounded to one decimal place. Additional precision would be false precision, given how much the input measurements themselves vary through the day.
Browser support. Any modern browser, desktop or mobile. No installation, no account.
Privacy: how this tool handles your measurements
The calculation happens in your browser using JavaScript already loaded with the page. Your height and weight are never transmitted, never logged and never stored. Closing the tab discards them completely, and there is no account, no history and no cookie recording what you typed.
This matters more than it might seem. Health measurements are personal data in the legal sense and sensitive data in the practical one. Plenty of health calculators are built as forms that post to a server, where the values land in request logs, analytics pipelines and error trackers that nobody intended as a health database but which function as one anyway.
Dividing one number by another does not require a server, so this tool does not use one. You can confirm it: open your browser's developer tools, watch the Network tab, and type your details in — nothing goes out. Or turn off your internet connection after the page loads. The calculator keeps working, because there is nowhere else for the work to happen.
Frequently asked questions
What is a healthy BMI?
The World Health Organization puts the healthy range at 18.5 to 24.9 for adults. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 and above as obese, subdivided into three classes. Those boundaries are statistical conventions drawn from population health data, not biological cliff edges — a BMI of 24.9 and a BMI of 25.1 describe essentially the same body, and moving across a line changes nothing about your health that day.
Is BMI accurate for muscular people?
No, and this is its best-known failure. BMI uses only weight and height, so it cannot tell muscle from fat. Muscle is denser than fat, which means a rugby player, a weightlifter or a sprinter routinely reads as overweight or obese on a scale that was never built for them. If you carry noticeably more muscle than average, BMI will overstate your risk and a body fat measurement or a waist reading will tell you far more.
Does BMI work the same for every ethnicity?
It does not. The standard thresholds were derived largely from European populations, and research has consistently found that people of South Asian, Chinese and other Asian backgrounds develop type 2 diabetes and cardiovascular disease at lower BMI values. Because of this, the WHO and the UK's NICE recommend lower action points for these groups — around 23 for overweight and 27.5 for obesity. If that applies to you, read your result against those figures rather than the standard ones, and talk to a doctor.
Can I use this for a child?
No. Children and teenagers are still growing, and their body composition changes rapidly with age and puberty, so a raw BMI number means nothing on its own. Paediatric assessment uses BMI-for-age percentile charts that compare a child against others of the same age and sex. Ask a GP or paediatric nurse rather than reading an adult figure across.
What is the BMI formula?
In metric it is weight in kilograms divided by height in metres squared. Someone who is 70 kg and 1.75 m tall calculates 70 divided by 1.75 squared, which is 70 divided by 3.0625, giving 22.9. The imperial version is weight in pounds multiplied by 703, divided by height in inches squared — the 703 is just the unit conversion folded into a single constant. This tool converts imperial input to metric internally and uses one formula, so both unit systems give the same answer.
Is BMI useful at all, given all the caveats?
Yes, for what it was designed to do. It was built as a population screening measure, and at that scale it works well and costs almost nothing to collect — two measurements anyone can take. For an individual it is a first pass: a rough flag that says whether a closer look is worth taking. The mistake is treating a screening flag as a diagnosis. Used as a starting point rather than a verdict, it is genuinely useful.
What should I look at alongside BMI?
Waist measurement is the most useful companion, because it captures where fat is stored, and fat around the abdomen carries more metabolic risk than the same amount elsewhere. A simple rule that holds across heights and ethnicities is to keep your waist under half your height. Beyond that, blood pressure, resting heart rate, blood glucose and cholesterol tell you far more about actual health than any weight-derived number.
Are my height and weight sent anywhere?
No. The calculation runs in your browser, on your device. Nothing is transmitted, logged or stored, and closing the tab discards it. Health measurements are exactly the sort of data that should not be casually posted to a server you know nothing about — and there is no technical reason for a division problem to leave your computer.