The Body mass index or BMI is the calculation applied when determining your body fat level. BMI can also help your doctor figure out your overall fitness, as well as the risk of getting chronic diseases. Furthermore, to be considered a healthy weight your body mass index should be between the range of 18.5 and 24.9 according to the National Heart and Lung Institute. If the BMI falls 18.5 it is considered underweight and if it is above 25.0 it is considered overweight or obese. The BMI number is important because if it falls in the low sector your body is malnourished, and it means that your body is not taking in the proper nutrients it is supposed to be taking. Or you are not getting the required calories your body needs to supply your activity level. On the other hand, if you have a high BMI it can cause heart disease, and can sometimes be cancerous.