BMI Calculator

What BMI measures, and what it does not

Body Mass Index is weight in kilograms divided by height in metres squared. That is the whole calculation. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was explicitly describing populations rather than assessing individuals — a distinction that matters enormously and gets lost constantly.

BMI does not measure body fat. It does not distinguish muscle from fat, or know where on your body the weight sits. It is a cheap screening ratio that correlates loosely with health risk across large groups, and it is used because measuring actual body composition is expensive and measuring height and weight is not.

BMI is a screening indicator, not a diagnosis. A number in any category is a prompt for a conversation with a clinician, never a conclusion on its own.

The categories

These cut-offs are conventions, not biological thresholds. Nothing changes in your body between 24.9 and 25.0. They exist so that population statistics can be reported consistently, and the sharp edges are an artefact of that.

Different thresholds for South and East Asian populations

Research has found that people of South Asian, Chinese and other East Asian descent tend to carry more visceral fat at a given BMI, and face raised cardiometabolic risk at lower numbers. The WHO has discussed lower action points for these groups — commonly cited around 23 for increased risk — and several national guidelines, including India's, use thresholds below the standard 25. If this applies to you, treat the standard bands as conservative.

Where BMI misleads

Muscular people

Muscle is denser than fat. A rugby forward or a serious lifter can post a BMI of 30 with low body fat, and the number will call them obese. This is the most cited failure of BMI and it is entirely real.

Older adults

Muscle mass declines with age while fat often increases. Someone can hold a stable weight, and therefore a stable BMI, while their body composition shifts substantially. The number stays still while the thing it is meant to proxy for does not.

It ignores where fat sits

This is the important one. Fat around the abdomen carries considerably more metabolic risk than the same amount on hips and thighs, and BMI cannot see the difference at all. Two people with identical BMIs can have meaningfully different risk profiles.

Children and pregnancy

Adult BMI categories do not apply to under-18s, who are assessed on age-and-sex percentile charts instead. BMI is also not a meaningful measure during pregnancy.

Measures that add what BMI misses

None of these replace clinical assessment, but each captures something BMI cannot, and all are measurable at home with a tape measure.

Waist-to-height is the easiest to remember and needs no chart: your waist should measure less than half your height. It applies across adult heights and both sexes.

Using the number sensibly

• Track the direction over months rather than fixating on a single reading.
• Weigh yourself at a consistent time — first thing in the morning is most repeatable, since daily fluctuation of one to two kilograms is normal.
• Pair BMI with a waist measurement. Together they say considerably more than either alone.
• If you strength train, expect BMI to overstate your risk, and rely on measurements and how clothes fit instead.
• Take any category boundary as a reason to ask a clinician, not as a verdict.

This guide is general health information, not medical advice. Discuss your own weight, risk factors and any plan to change them with a qualified healthcare professional.