BMI stands for Body Mass Index, a number calculated from a person’s mass and height to describe relative body size.
The metric is calculated by dividing body mass in kilograms by height in metres squared: kg/m². For example, doubling height changes the denominator substantially because height is squared. 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 above as obese.
The idea behind BMI was developed by Belgian mathematician and astronomer Adolphe Quetelet between 1830 and 1850. He called it the Quetelet Index and created it as part of his statistical study of the “average man,” not as a direct medical test of an individual’s body fat.
The modern name “body mass index” was introduced by Ancel Keys and colleagues in a 1972 paper. BMI remains useful for population studies and initial screening, but it does not distinguish fat from muscle, bone, or differences in fat distribution. Waist circumference and other measures can provide important additional context.