Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems. BMI does not calculate body fat. It does not change based on whether you are a man or woman, age or activity level. The higher your BMI above acceptable ranges, the greater your risk of health problems such as diabetes, heart disease, some cancers, and high blood pressure.