Body Mass Index, or BMI, is used as an inexpensive and easy to calculate measure of body fat, according to the Centers for Disease Control and Prevention, CDC. It is determined by dividing weight by height and is used by healthcare professionals to compare a teen's growth to others of the same age and gender. If the teenager's BMI falls outside of the healthy weight percentile, a doctor will determine if diagnostic evaluation is needed.
Calculating BMI
BMI is calculated by using a standard formula. Weight in pounds is divided by height in inches squared then multiplied by 703. While the formula is the same for children and adults, the way it is interpreted differs. For youth under the age of 19, the BMI is then put into age and gender specific percentiles and placed on the age growth charts.
Adult vs. Teenagers
The interpretation of BMI for children and teenagers is dependent upon age and gender for two reasons, according to the CDC. First, the amount of body fat in a child changes with age and second, the amount of fat in the body is different for boys and girls. To allow for these variations, the BMI number is translated into percentile and compared to other children of the same age and sex. For adults, the BMI number is grouped into categories that are not dependent upon age and gender. This is then converted into healthy weight ranges based on height.
Percentiles
Healthy weight ranges cannot be determined for teenagers because these would change monthly based on gender. Healthy weight also changes as the teen grows taller. The CDC uses the BMI age and sex percentile to interpret the BMI number. This is charted on a growth chart which determines if the teen is underweight, overweight or in the healthy weight range. A teen is underweight if his BMI falls in the fifth percentile, healthy if the BMI is between the fifth and the 85th percentile and overweight between the 85th and 95th percentiles. Teenagers are obese if their BMI falls above the 95th percentile.
Limitations
It is possible for a teenager to have a high BMI but not have excess fat. The same limitation applies to adults. Body weight is used in the calculation which measures muscle as well as fat. The American Council on Exercise states that muscle weighs more than fat, so athletes and other muscular builds may fall into the overweight or obese categories without having excess body fat. Other body composition tests -- like the skinfold method -- may need to be used to fully evaluate health risks.
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