BMI divides mass by height squared, and people are not flat. Scale a body up in every direction and keep the proportions identical: the mass follows the volume and grows with the cube, while the height grows only once. The index therefore rises in step with height for two people built exactly the same way.
Mass divided by height squared, kg ÷ m². Adolphe Quetelet devised it in the 1830s to describe the “average man” across a population, not to assess a patient.
WHO adult bands: under 18.5, normal to 24.9, overweight to 29.9, obese from 30 (classes at 35 and 40). Asian cut-offs are lower, at 23 and 27.5.
It tracks body-fat percentage across a population, cheaply, with scales and a tape. For one person it cannot tell muscle from fat.
50 kg + 2.3 kg per inch over 5 ft (45.5 kg for women). Built for scaling drug doses, not as a weight goal.
South and East Asian populations show higher diabetes and cardiovascular risk at BMIs the standard scale calls normal, so several national bodies use the lower cut-offs.
A rugby prop or a bodybuilder can carry a BMI over 30 with single-digit body fat. The formula only sees total mass, so muscle and fat look the same to it, and that is its most common false positive.
Next to waist-to-height, blood pressure or a direct body-composition measurement, BMI alone says little about an individual. At population level, higher bands do track higher rates of diabetes and heart disease.
Body mass scales roughly with volume, and volume scales with the cube of a linear dimension like height — but weight-bearing area and practical measurement made height-squared the better-fitting compromise Adolphe Quetelet settled on in the 1830s. It's an empirical fit to population data, not a law of physics.
BMI only ever sees total mass and height — it has no way to distinguish a kilogram of muscle from a kilogram of fat. A muscular, lean athlete can post an "overweight" BMI despite having very low body fat, which is the single most common false positive the number produces.
Reasonably well within the normal adult height range, which is why it remains useful — but it systematically overestimates fatness in very tall people and underestimates it in very short people, a known distortion from the height-squared approximation not being perfect.
No — the WHO standard thresholds shown here are calibrated for adults of European descent. Several national health bodies use lower overweight/obese cutoffs for South Asian, Southeast Asian and Chinese populations, since health risk at a given BMI is measurably higher in those groups at the same number. Toggle "Asian BMI cutoffs" above to apply the WHO Asian bands (overweight ≥23, obese ≥27.5).
Devine (1974) estimates ideal body weight from height and sex: for men, 50 kg plus 2.3 kg per inch over 5 feet; for women, 45.5 kg plus 2.3 kg per inch over 5 feet. It was developed for drug dosing, not body-composition goals — treat it as a rough reference, not a target.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.