A body mass index chart illustrates the body's measure of body fat based on height and weight, according to the National Institutes of Health. The chart shows BMI categories of underweight, <18.5; normal weight, 18.5–24.9; overweight, 25–29.9; and obese, a BMI of 30 or higher.
Body mass index charts provide a reliable measure of a person's body fat that can be used to assess and screen for health issues related to body weight, states the Center for Disease Control and Prevention. BMI does not measure fat directly, but it correlates to direct measures of body fat. BMI is not used as a diagnostic tool, but instead as a screening for health risks. The formula for calculating BMI is: weight (lb) / [height (in)]2 x 703.