How to use this BMI calculator
Type your weight in kilograms into the first box. Type your height in centimetres into the second. Your score updates as you type. There is no button to press.
Working in pounds and feet? Divide your pounds by 2.205 to get kilograms. For height, multiply your total inches by 2.54. So 5 ft 7 in is 67 inches, which lands at about 170 cm.
What is BMI and how is it calculated?
Take your weight in kilograms. Divide it by your height in metres, squared. That is the whole formula.
Say you weigh 70 kg and you are 1.75 m tall. You divide 70 by 1.75 times 1.75. Your BMI is 22.9.
This tool asks for centimetres instead of metres, because nobody thinks of their height as 1.75. It divides by 100 for you.
Why do health services use this at all? Speed. You need a scale, a tape measure, and nothing else. No lab, no appointment, no equipment. That makes it a decent way to look at a thousand people at once. Whether it says anything useful about you personally is a much harder question, and the rest of this page gets into why.
BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
The World Health Organization set these bands, and they cover most adults aged 20 and over. Kids and teens are scored a completely different way, against age and sex percentile charts. An adult band tells you nothing about a 12-year-old.
Why the standard cutoffs do not fit everyone
Those 25 and 30 lines were drawn mostly from European data. For a big share of the world, they are set too high.
Back in 2002, a WHO expert panel recommended lower trigger points for Asian populations: 23 to 27.5 counts as overweight, and 27.5 or above as obese.
The reason is simple. At the same BMI, Asian adults tend to carry more fat, and more of it around the organs. They also develop type 2 diabetes at lower numbers than the standard bands predict. The International Diabetes Federation, the American Diabetes Association, and most Asian health authorities now use those lower cutoffs.
Here is what that means in practice. If you are of South Asian, East Asian, or Southeast Asian descent, a BMI of 24 is not sitting comfortably mid-range. It is near the top.
Where this number came from
A Belgian mathematician built it, not a doctor. Adolphe Quetelet worked it out somewhere between 1830 and 1850, and for over a century people called it the Quetelet Index. His 1835 book A Treatise on Man showed that across a whole population, weight tends to scale with height squared. He built it to describe crowds, not patients. Quetelet said plainly that a statistical pattern describes a group, not any one person standing inside it, and he never once pitched it as a health check.
The name you know arrived 137 years later, when the American physiologist Ancel Keys published a 1972 paper in theJournal of Chronic Diseases and called it the Body Mass Index. Doctors picked it up through the 1970s and 1980s. So a crowd statistic from the 1830s ended up as a number in your medical file. Most of what goes wrong with it traces straight back to that.
What the number cannot tell you
Muscle and fat do not weigh the same, and this formula cannot tell them apart. Stand a rugby player next to someone who never trains. Same height, same weight, same score. That is exactly why so many athletes get filed under overweight. The formula is also blind to where you carry it, which matters a lot, because fat around your middle carries different risk than fat on your hips and thighs.
In June 2023 the American Medical Association took a formal position. Its House of Delegates ruled that using BMI on its own is an imperfect clinical measure, noting it ignores differences across racial and ethnic groups, sexes, and ages, and that the whole thing was built on data from earlier generations of non-Hispanic white people. They did not tell doctors to bin it. They said pair it with something real: visceral fat, body composition, relative fat mass, waist size, genetic and metabolic factors.
Read your score as a starting point for a conversation. Not a verdict. It was never designed to be one.
Medical disclaimer: This calculator is general information, not medical advice. A BMI score cannot diagnose anything or judge your personal risk. Talk to a doctor or a qualified healthcare professional before you make decisions about your weight, diet, or health.
Frequently asked questions
What counts as a good BMI?
For most adults, anywhere from 18.5 to 24.9. If you are of Asian descent, the WHO band puts the ceiling closer to 23. Either way, the score only compares your weight to your height. It cannot tell you whether you are healthy.
Is BMI accurate for athletes?
No. Muscle is denser than fat, so a trained body posts a high score while carrying very little fat. Bodybuilders and rugby forwards regularly land in the obese band. If you lift seriously, look at body fat percentage or waist measurement instead.
Does BMI work for children?
Not with these bands. Kids and teens get scored on percentile charts for their age and sex, because a growing body's healthy ratio shifts year to year. Use a paediatric chart from a health authority.
Why does my BMI differ between calculators?
Rounding. Some tools round your height to the nearest inch before converting, some keep more decimal places, and a few run the old imperial formula with the 703 multiplier. The gap is usually under 0.3. That only shifts your category if you were sitting right on a line anyway.
Should I use a different BMI scale for my ethnicity?
If you are of Asian descent, yes. The 23 and 27.5 cutoffs are the ones the WHO and the major diabetes bodies back. Other groups still use the standard bands, though the AMA position from 2023 is that this measure is weak on its own for everybody.
Related: estimate your daily calorie burn · ideal weight ranges · body fat percentage · daily water intake.