Calculate your Body Mass Index (BMI). BMI is an estimate of body fat based on height and weight. For most folks its a good starting point for determining body fat percentage. BMI is an estimate, not an exact measurement, because it does not take into account an individual's exact muscle mass, bone density, overall body composition, and race and gender. So for example, an elite athlete could have a BMI rated as "obese" because of their significantly increased proportion of muscle weight.