This body mass index calculator for adults is actually 4 calculators in 1. In addition to Body Mass Index, you can calculate your Body Surface Area, Ideal Body Weight, and Lean Body Weight. Body Mass Index (BMI) provides a simple and useful estimate of overweight, obesity, and body fat distribution by determining your weight relative to your height. This calculator helps you to know if your weight is above normal (overweight or obese) and use frequently if you are trying to lose weight.