Body Mass Index (BMI) Calculator

BMI was devised in the 1830s by Adolphe Quetelet as a population-level statistic, not a diagnostic tool for individuals. It cannot distinguish muscle from fat, so a bodybuilder and a sedentary person of the same height and weight can share an identical BMI while having very different body compositions. The Deurenberg equation used here compensates by adding age and sex terms - body fat rises with age even when BMI stays fixed, and men carry more lean mass than women at the same BMI. Even so, no formula built from height, weight, age, and sex alone can replace direct measurement, such as a DEXA scan, skinfold calipers, or bioelectrical impedance, for an individual's true body composition.

The BMI answers one simple question: is your weight in proportion to your height? It divides your mass in kilograms by your height in meters squared, so people of very different heights land on one comparable scale. The practical payoff is that every height has its own healthy weight window — this tool computes yours in kilograms, the span where BMI stays between 18.5 and 25.

Weight tracks volume, and volume grows with length cubed, so pure geometry says BMI should divide by height cubed — the ponderal index this tool also shows. Quetelet kept the square because real bodies don't scale like enlarged statues: taller people are relatively more slender, so height squared fits whole populations better. That mismatch is why BMI slightly overstates tall people and understates short ones, and why pediatricians reach for the ponderal index with infants.