The body mass index, or BMI, is used to estimate roughly how overweight or underweight someone is. A person's BMI is calculated by taking his weight (in kilograms) and dividing it by the square of his height in meters. The BMI can be calculated using other units of measurement, such as pounds and ounces and feet and inches, by applying correction factors to convert the measurements to the metric system.
The US Centers for Disease Control and Prevention uses the BMI to make rough recommendations on how your weight for your height may influence your health. For adults and older adolescents, BMIs of 19 to 25 are regarded as "average". A person with a BMI of 25 to 30 is considered overweight, while someone whose BMI is over 35 is considered obese. On the other end of the spectrum, a BMI of less than 18 may indicate malnutrition, and a BMI of less than 17.5 is one of the American Psychiatric Association's criteria for the diagnosis of anorexia nervosa.
As with weight, height, and the rate of change in each, average BMI ranges are different at different ages. Children in their preteen years have much lower average BMIs than do small children or adults. The CDC's National Center for Health Statistics, which publishes the standard growth charts that pediatricians use to keep track of how children are growing, also publishes standard curves for BMI in boys and girls younger than 18 years. We can use these curves to find out how a particular child's BMI compares to average; this information may alert us and our patients' parents to a possible obesity or anorexia problem much earlier than we might notice just by examiniation.
Again, the BMI is a rough estimate of weight and stature. Just looking at weight and height doesn't tell us whether the extra weight is from fat or muscle. Many trained athletes including football players have high BMIs, but their "extra bulk" tends to be muscular.