BMI is based on weight and height, not on body fat.
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The body mass index, or BMI, is a metric used to estimate the amount of body fat a person has.
Though BMI doesn't measure body fat directly, it correlates with other direct measures of body fat, according to the Centers for Disease Control and Prevention (CDC).
Only two measurements are involved in BMI: height and weight. To calculate your BMI using pounds and inches, divide your weight by your height squared, and then multiply by the conversion factor of 703. (The calculation is the same using kilograms and meters, except no conversion factor is needed.)
For example, a person who weighs 130 pounds and is 5 foot, 6 inches (66 inches) tall would have a BMI of 20.9: 130 / (66 x 66, or 4,356) x 703 = 20.9.
On the BMI scale, anything below 18.5 is considered "underweight," 18.5 to 24.9 is "normal," 25.0 to 29.9 is "overweight" and an index of 30 or more is "obese."
For example, studies have shown that people with BMIs of 30 or more have increased risk of death from a number of diseases, including heart disease, diabetes and colorectal cancer. There are also issues with being underweight on the BMI scale, such as increased risk for malnutrition, osteoporosis and anemia.
In recent years, many experts have expressed their doubts about relying too much on the body mass index, stressing that it is not an accurate measure of body fat or health.
One reason is that BMI does not take into account age and sex. Women tend to have more body fat than men of the same BMI, and older people tend to have more body fat than younger people of the same BMI, according to the CDC.
Additionally, BMI has no way of measuring where body fat is located in the body. Studies have shown that belly fat — fat around the abdominal organs — is far more dangerous than the peripheral fat beneath the skin in other areas of the body.
Finally, BMI doesn't consider bone density or muscle mass — highly trained athletes may have high BMIs because of increased muscularity.
To correct for the index's shortcomings in determining a patient's risk of disease, the National Heart, Lung, and Blood Institute recommends using two other predictors in addition to BMI: waist circumference and risk factors associated with obesity, such as high blood pressure and physical inactivity.