Index, body mass (BMI): An index that relates body weight to height. The body mass index (BMI) is obtained by dividing a person's weight in kilograms (kg) by their height in meters (m) squared.
The National Institutes of Health (NIH) now defines normal weight, overweight, and obesity according to the BMI rather than the traditional height/weight charts. Since the BMI describes the body weight relative to height, it correlates strongly (in adults) with the total body fat content.
Overweight is a BMI of 27.3 % or more for women and 27.8 % or more for men, according to the NIH.
Obesity is a BMI of 30 and above, according to the NIH. A BMI of 30 is about 30 pounds overweight. Some very muscular people may have a high BMI without undue health risks.