Short description of Body Mass Index Calculator 1.1: BODY MASS INDEX (BMI) is a single number which expresses a relationship between height, body surface area, and body weight. The BMI is very useful because it correlates very closely to your body fat percentage which is much more difficult to measure. This simple BMI tool will calculate your BMI and tell you your "weight status" according to the Panel on Energy, Obesity, and Body Weight Standards.