BMI

BMI(Body Mass Index) is a single number summarising the relationship between someone’s weight and height. It’s computed as weight ÷ height²in kilograms and meters. The imperial form multiplies by 703: BMI = 703 × weight (lb) ÷ height (in)².

Devised by Belgian statistician Adolphe Quetelet in the 1830s and renamed BMI by physiologist Ancel Keys in 1972. The World Health Organization uses BMI as the primary population-screening tool for under- and over-weight, though it’s well known to misclassify individuals with atypical body composition (athletes, the elderly, children).

Standard WHO category bands: < 18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, ≥ 30.0 obese (subdivided into classes I-III).

See our BMI calculator for the live tool, or our guide on BMI accuracy for when it misleads.