The Body Mass Index is just one of the tools to identify the severity of obesity in an individual. It is used more for screening, rather than diagnostic purposes. BMI is a simple index of weight for height that is commonly used to classify overweight and obesity in adults. It is calculated as a person's weight in kilograms divided by the square of their height in meters (kg/m2).

BMI in the normal range however does not always mean one is healthy. For e.g. one may have BMI in normal range with too less muscle mass and majority of fatty tissue. Also, it is not applicable to pregnant females and breast feeding mothers. Unusual muscular mass, such as that found in trained athletes, can raise BMI to 30, but seldom to 32. 

BMI classifications and cut-offs are frequently used to guide patient care.