Calculate your BMI with our free BMI Calculator to determine the amount of body fat and amount of lean body mass. BMI can be used to determine if people are at a healthy weight, overweight, or obese.The BMI is meant to broadly categorise populations for purely statistical purposes.It's accuracy in relation to actual levels of body fat is easily distorted by such factors as fitness level, muscle mass, bone structure, gender, and ethnicity. People who are mesomorphic tend to have higher BMI numbers than people who are endomorphic, because they have greater bone mass and greater muscle mass, respectively, than do endomorphic individuals.