Body mass index (BMI), which measures weight in relation to height, is used to determine whether a child is overweight. A child with a body mass index (BMI) at or above the 95th percentile on a growth chart is usually considered overweight.

In some cases, a child may be overweight because he or she has a large amount of body fat (adipose tissue). However, not all children with BMIs greater than the 95th percentile have too much body fat. For instance:

• A child who has grown consistently at a higher percentile for most of his or her life may just be bigger than other children of the same age due to genetics.
• Before and during puberty, it is normal for children to have a significant gain in weight before beginning to grow in height. This can temporarily increase a child's BMI.
• Children who are very muscular (for instance, children who are very active in sports) may have a higher BMI but have normal or even low amounts of body fat.