Body Mass Index (BMI) is a proxy for human body fat based on an individual's weight and height. The human body consists of many different tissues ranging from muscle, to fat and bone. Each tissue weighs differently muscle weigh more than fat, and bone more than muscle. To correctly determine body composition one need highly advanced instruments, but to get an estimation of the relation between weight and height one can use BMI. Do not over-interpret BMI! Regular BMI is not recommended for children and adolescents under 18 years of age. Also, BMI should be interpret with caution in well-fitted people. The calculates BMI using metric or US units.