Body Mass Index (BMI)

Body mass index (BMI) is a number calculated from weight and height: weight in kilograms divided by height in meters squared. It takes seconds, costs nothing, and needs no equipment beyond a scale and a tape measure, which is why it appears in nearly every routine health visit.

BMI does not measure body fat directly. It is associated with body fat but cannot distinguish muscle from fat, tell where fat sits on the body, or account for how the relationship shifts with age, sex, and ethnicity. Public health agencies describe it as a screening measure, not a diagnostic one — a starting point that should sit alongside blood pressure, lab results, and physical examination. For children and teens, BMI is read as a percentile against growth charts for the same age and sex.

• CDC's overview of BMI, its strengths, and its limits

Learn More: Adult categories are underweight, healthy weight, overweight, and obesity, applied the same way regardless of age or sex. Children are different: BMI-for-age percentile is used instead, with underweight below the 5th percentile and obesity at or above the 95th.