The body mass index (BMI) is a statistical measurement which compares a person 's weight and height. Though it does not actually measure the percentage of body fat, it may be a useful tool to estimate a healthy body weight based on how tall a person is. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify weight problems. By using BMI calculator you can find out if your weight is normal or you need to gain flesh or start to diet to grow slim.