Body mass index is calculated by dividing a person's weight by her height. The resulting number is found on the BMI chart to estimate whether a person is in a healthy weight range, according to the Centers for Disease Control and Prevention.
A person with a BMI value under 18.5 is considered underweight, while a person with a BMI between 18.5 and 24.9 is considered normal weight. Anyone with a BMI between 25 and 29.9 is considered overweight, and a person with a BMI over 30 is considered obese. The values are sometimes misleading. For example, an athlete might have a higher BMI because of increased muscle mass, notes the Centers for Disease Control and Prevention.