In the BMI formula, body mass is divided by the square of height.
BMI expresses mass relative to height: in metric form, it is kilograms divided by metres squared (kg/m²). Squaring height makes the index adjust for body size more effectively than simply comparing weight. In US customary units, pounds and inches are used with a conversion factor of 703.
The method traces back to Belgian statistician Adolphe Quetelet, who developed the underlying index between 1830 and 1850. The modern name “body mass index” was popularized by Ancel Keys and colleagues in a 1972 paper.
BMI is a screening measure, not a direct reading of body fat. Adult categories commonly label values below 18.5 as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 or higher as obese. Muscular athletes can receive misleadingly high values because BMI cannot distinguish muscle from fat.