BMI Calculator
Calculate body mass index from height and weight.
BMI was designed for populations, not people
The formula dates to the 1830s, when Adolphe Quetelet was looking for a way to describe the average build of a population — not to assess any individual in it. Weight divided by height squared happens to correlate reasonably well with body fat across large groups, which is why it survives in epidemiology. Applied to one person it carries no information about body composition at all, because it cannot distinguish a kilogram of muscle from a kilogram of fat.
This is not a minor caveat. A well-trained athlete frequently lands in the "overweight" band while carrying very little fat, and someone sedentary with low muscle mass can sit in the "normal" band while carrying a great deal. The number is a screening flag, not a diagnosis.
Where the standard categories break down
The cut-offs — under 18.5, 18.5 to 25, 25 to 30, above 30 — were set primarily from data on European-ancestry adults. Several health bodies now use lower thresholds for people of South Asian, Chinese, and other ancestries, because cardiometabolic risk begins to rise at a lower BMI in those populations. The categories also perform poorly at the extremes of height, since the height-squared term systematically overestimates for tall people and underestimates for short ones.
BMI is not validated for children and adolescents, who need age-and-sex percentile charts instead; nor for pregnancy, nor for anyone with substantially altered body composition through illness, amputation, or heavy training.
What to look at alongside it
Waist circumference and waist-to-height ratio track abdominal fat, which is the fat most strongly associated with metabolic risk, and both are easy to measure at home. A waist-to-height ratio below 0.5 is a common rule of thumb. Beyond that, blood pressure, resting heart rate, blood lipids, and fasting glucose tell you far more about health than any body-size index — and unlike BMI they respond visibly to changes in activity and diet even when weight barely moves.
How to use it
- Enter your height in centimetres and weight in kilograms.
- Read the value as a rough position within a population range, not as a verdict.
- Measure your waist as a second data point — it is more informative for metabolic risk.
- Discuss anything concerning with a clinician who can interpret it against your full picture.
Common questions
Is BMI accurate for athletes?
No. Muscle is denser than fat, so a lean, muscular person routinely scores in the overweight or obese range despite low body fat. BMI cannot distinguish tissue types, which is its central limitation for individual assessment.
Do the categories apply to everyone equally?
No. The standard thresholds were derived mainly from European-ancestry populations, and several health bodies recommend lower cut-offs for people of South Asian and Chinese ancestry, where metabolic risk rises at a lower BMI.
Can I use this for a child?
No. Children and adolescents are assessed with age-and-sex-specific percentile charts, not adult BMI categories. A paediatric clinician should interpret those charts.
What is a better measure to track?
Waist circumference or waist-to-height ratio reflects abdominal fat more directly and is simple to measure at home. Blood pressure, resting heart rate, and blood markers tell you more about health than any size index.
Disclaimer: These estimates are general information, not medical advice, diagnosis, or treatment. Population formulas do not account for individual physiology, medication, or medical conditions. Consult a qualified clinician about your own health, and seek immediate care for any urgent symptom.