BMI stands for Body Mass Index, and it is a tool used to determine if a person is overweight and to what degree. It is a calculation based on height and weight that can provide a better estimate of total body fat than weight alone. BMI is a screening measure and is not intended to diagnose disease or illness. The healthy BMI range for adults is between 18.5 and 24.9. However, BMI does not differentiate between body fat and muscle mass, so it may not be an accurate measurement of body fat for bodybuilders, certain athletes, and pregnant women. In adults, BMI does not correlate to age, but certain calculators may give you the average weight for a set of age ranges. BMI is used slightly differently in children, in that it is calculated the same way as it is for adults, but then the ranges are compared to percentiles for children of the same age and sex.