Body mass index (BMI) is one way to estimate a person's body fat that takes into consideration the person's height. BMI is calculated using a person's weight and height. In children and teens, BMI is used to find out if a child or teen is underweight, of a healthy weight, overweight, or obese. A child's body fat changes with age. Also, girls and boys differ in their amount of body fat as they mature. This is why BMI for children, also known as BMI-for-age, includes gender and age.