At a glance Body Mass Index (BMI) is a calculated measure of body weight relative to height. Because children and teens are growing, their BMI values are expressed relative to others of the same sex and age. These values are called BMI-for-age percentiles. Child and teen BMI categories are based on these percentiles, and include underweight, healthy weight, overweight, obesity, and severe obesity.

​https://www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html