The body mass index (BMI) is a measurement used to determine relative body fat. The number will indicate if a person is normal weight, underweight, overweight, or obese. The BMI measurement uses weight and height in its calculation and is therefore not a method that directly measures fat.

Other methods, including underwater weighing and skin fold thickness measurements, are more accurate but are also more costly. BMI provides a reliable approximation and will indicate if an individual has serious body fat issues. Then, the lifestyle can be adjusted accordingly.

To measure your BMI, multiply your weight (in pounds) by 703, then divide by your height (in inches) twice:

weight (lbs.) × 703 / height (inches) / height (inches)

Keep in mind that BMI is affected by several variables, including age, sex, race, and musculature. Older adults and women tend to have more body fat, while athletes tend to have heavy muscles.

For that reason, a 20-year-old male athlete may have the same BMI as a 40-year-old female office worker, but they may be miles apart in terms of actual fitness. The measurement is meant only as a general indication.