Body Mass Index (also known as BMI) is a number calculated from a child’s weight and height. BMI is a reliable indicator of body fatness for most children and teens. BMI is not an exact measurment of body fat, but research has shown that it does correlate to direct measures of body fat. BMI can be considered an alternative for direct measures of body fat. BMI is also an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.