The Body Mass Index, or BMI, can be used for both children and adults. For adults, BMI is measured via a calculation using the person's weight and height. For children between the ages of 2 and approximately 19, the BMI calculation is more complex.
When measuring the BMI of someone under the age of 20, the BMI measurement and the age of the child/teen is plotted on a growth chart, to get a percentile ranking. The percentile ranking is used to determine where the child/teen fits in the BMI range of other children/teens of the same age. From that point, it can be determined whether the child is under- or overweight, or in a healthy weight range.