The formula for body mass index for children and teenagers is their weight in kilograms divided by their height in meters squared, according to the Centers for Disease Control and Prevention (CDC). However, unlike adults, children have their BMI compared to one another to determine whether their weight is appropriate for their height.
Once the BMI is calculated, it is placed in a percentile chart of either boys or girls in the United States, provided by the CDC. Children are growing and developing, so their weights and body fat indexes are evaluated relative to others in their same height and weight.
For children, the BMI is not a diagnostic tool and is intended to be used as a screening tool for weight-related health problems, according to the CDC. To determine if a child has too much body fat, a doctor must perform other tests, such as a skinfold measurement. The BMI can be used in children ages two and up.
Children in the 5th to 85th percentile in the United States are considered a healthy weight, reports the CDC. Children in the 95th percentile are considered obese; or, if a child weighs more than 95 percent of the rest of the children in the United States, he is obese. Children and teenagers do not have a healthy weight range as provided by the CDC due to the number of factors that influence BMI, such as age and sex.