BMI is nothing but Body mass index. If you know the weight in kgs and height in metres, then we can calculate the Body mass index. Body mass index is calculated by weight divided by height square. So the normal Body mass index is in between 18.5 and 25, then it is ideal weight. If it is below, 18.5 kg per metre square, then it is underweight. If it is above 25 kg per metre square, then you are overweight. So the ideal wright is measured by body mass index. Body mass index or BMI is calculated by weight divided by height square. So the normal body mass index is in between 18.5 and 25. So it measures the body composition of muscles, bones and musculoskeletal system put together and with the fat. So the amount of fat also we get to know with this body mass index. So we get to know if the person is overweight or if he is underweight according to his height and also get to know the amount of musculoskeletal system in the body. So it tells us what is the ideal weight of the individual and if he is overweight or underweight and he requires and treatment or if he suffers from any diseases because of the weight problems.