The body mass index (BMI) for teens is the relationship between the height and weight of the teen or the child, which directly correlates to the body fat contained within the body and explains the degree of health risk associated with the health of the person. It is measured as BMI = body weight in kilograms divided by height in meters squared.

In the case of children and teens, the BMI, also called as BMI-for-age, cannot always be consistent. Since it is based on the height and weight of the child or teen, and they continuously grow, their amount of body fat also changes continuously as they mature. Also, teen boys grow at a different rate than teenage girls do. That is the reason that the BMI tends to be age and gender specific and is not always reliable for children and teens.

The guidelines for body types for children and teens are usually done with respect to an important statistic called BMI percentile. This statistic compares the person's BMI to thousands of other people under the same reference sample, so as to enable a comparative study of that person's health with respect to the rest of the population within permissible constraints. People are considered to be underweight if they are in the lowest 5% of the people studied and overweight if they are in the top 5%. People are considered to be in the normal weight range if they are in the middle 90%.

The BMI-for-age is a useful tool because it can be used for adolescents and children even beyond puberty. It compares well with the laboratory results of the body fat and can be used to keep a track of the body size through-out the child's or the adolescent's life.