Body mass index, or BMI, provides a good estimate of how much body fat a child or teen has. Although it does not provide a definitive diagnosis of a weight problem, it can indicate when further evaluation might be needed. The calculation, which requires knowing the child's weight and height, is simple. Interpreting the result requires the use of tables or charts provided by the Centers for Disease Control and Prevention.
Calculating BMI for Children and Teens
The formula for calculating BMI for people 19 years or younger is the same as that used for adults. If you know a child's weight in kilograms and height in meters, just plug the numbers into the formula: BMI = weight ÷ [height in meters x height in meters].
If you know the child's weight in pounds and height in inches, then use the formula: BMI = [(weight in pounds) ÷ (height in inches x height in inches)] x 703.
Interpreting BMI for Children and Teens
Although they are calculated in the same way, BMI values for adults and children are evaluated differently. Adults use a simple table which lists ranges of BMI values and categories of weight status ranging from underweight to obese. Placing a child's BMI into proper context involves what the CDC calls "BMI-for-age." This is necessary because differences in sex and small differences in age greatly influence body fat and its health implications in children. In other words, "BMI is age- and sex- specific," according to the CDC.
BMI-for-Age Growth Charts
Because, as the National Heart, Lung, and Blood Institute website points out, "children are still growing, and boys and girls develop at different rates," you will need the CDC's BMI-for-age growth charts to determine the significance of a child's BMI. There are separate charts for boys and girls. These charts will tell you "the relative position of the child's BMI number among children of the same sex and age," according to the CDC. This relative position is known as a percentile ranking. Alternatively, you can use tools provided by the CDC to calculate a child's BMI, percentile ranking and weight status.
Using BMI as a Screening Tool
Once you know the percentile ranking, you can use the following guide provided by the CDC and National Heart, Lung, and Blood Institute to decide, in consultation with a physician, if a child might need to gain or lose weight: Underweight: < 5th percentile; Healthy: 5th to < 85th percentile; Overweight: 85th to < 95th percentile; and Obese: > 95th percentile.
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