What does the abbreviation "BMI" stand for in fitness and health?

In fitness and health, BMI stands for Body Mass Index. It is a numerical ratio that compares a person’s body mass with their height, using the formula mass in kilograms divided by height in metres squared. The result is expressed in kilograms per square metre, although it is commonly treated as a unitless number.

BMI has been used as a convenient screening measure for broad weight categories. For adults, commonly used ranges classify a BMI below 18.5 as underweight, 18.5–24.9 as the usual “normal” range, 25–29.9 as overweight and 30 or higher as obese. These thresholds describe population patterns rather than diagnosing an individual’s health.

The measure’s origins lie with Belgian mathematician and statistician Adolphe Quetelet, who developed the underlying ratio in the nineteenth century and called it the Quetelet Index. The modern term “body mass index” was popularised in a 1972 paper by Ancel Keys and colleagues.

BMI is often misunderstood as a direct measurement of body fat. It does not distinguish fat from muscle, bone or water, so athletes and muscular people may receive misleadingly high values. Waist measurements, medical history and other assessments can provide important additional context.