BMI Calculator

Body mass index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by Adolphe Quetelet, a Belgian statistician studying the distribution of measurements across populations, not a physician, and not studying individuals. That origin explains both its usefulness and its limits: it is a good instrument for comparing groups and a blunt one for describing a person. The World Health Organization thresholds in wide use today (under 18.5, 18.5 to 24.9, 25 to 29.9, 30 and above) are population reference ranges, not individual diagnoses.

The formula's central weakness is that it cannot distinguish what the mass is made of. Muscle is considerably denser than fat, so a trained athlete routinely lands in the "overweight" band while carrying very little body fat, and an inactive person of the same height and weight can sit in the same band for entirely different reasons. It also ignores where fat is distributed, which is the part with the clearest link to metabolic risk: waist circumference and waist-to-height ratio both track that better than BMI does.

The thresholds are also not universal. Research on South Asian, Chinese and other populations has found elevated metabolic risk at lower BMI values than the WHO cut-offs imply, and several health authorities publish lower action points for those groups as a result. BMI remains widely used because it needs only a scale and a tape measure, which is valuable at population scale. Read the number here as one input among several, and take any question about your own health to someone who can measure more than two things.