How to calculate BMI—and what the number really means
Body mass index, or BMI, is a screening measure that relates weight to height. It is widely used because it is quick to calculate and offers a common starting point for discussing weight-related health risks in adults. It should not be treated as a diagnosis or a complete picture of health.
The BMI formula
In metric units, BMI equals weight in kilograms divided by height in meters squared. In U.S. customary units, multiply weight in pounds by 703, then divide by height in inches squared. A calculator avoids the conversion and arithmetic, but the measurement depends on accurate height and weight inputs.
What BMI does not measure
BMI does not measure body fat directly. A muscular athlete and a person with little muscle can have the same BMI while having very different body composition. It also does not reveal blood pressure, cholesterol, blood sugar, sleep, diet quality, fitness, or family history.
Interpretation can differ for children and teens, who use age- and sex-specific growth charts. Pregnancy, older age, disability, medication, and a person’s individual health history can also change what a useful discussion looks like. For these reasons, BMI works best as one data point among several.
Use it as a starting point
Track a trend rather than reacting to one reading. If you are concerned about weight, nutrition, or a health change, a clinician can consider BMI alongside waist measurement, medical history, laboratory results, activity, and your goals. Do not use this article to replace medical care.