What is body mass index?

Body mass index (BMI) is a measurement using weight and height that gives an estimate of body fat. It's an easy way to screen for weight categories that may lead to health problems. BMI is a screening tool and does not diagnose body fatness. Or the health of an individual. If your child or teen has a high BMI for their age and sex, a healthcare provider may do more evaluations.

Although BMI is not a perfect formula or measurement, it gives a general idea of how much of your child or teen's body is composed of fat. BMI for children and teens is based on weight and height. So it's only an estimate of body fat. Two people may have the exact same BMI, yet have different amounts of actual body fat. BMI does not consider race or ethnicity.

For children and teens ages 2 to 19 years, the BMI varies by age and sex. An additional step must be done after the BMI has been determined using one of the formulas below. The BMI-for-age percentile is determined by comparing your child or teen's weight to that of other children or teens of the same age and sex.

In other words, by plotting your child's BMI value into the  CDC's BMI-for-age growth chart , you can determine if your child is underweight (below the 5th percentile), within normal range (5th to less than 85th percentile), overweight (85th to less than 95th percentile), or obese (at or above the 95th percentile). Or you can easily calculate it from the CDC's online tool.