BMI is a ratio of a person's weight to height. BMI is commonly used to classify weight as "healthy" or "unhealthy".

For most people, BMI provides a good measure of obesity. However, BMI does not provide actual information on body composition (i. e. the proportions of muscle, bone, fat and other tissues that make up a person's total body weight).

a) For an adult:
BMI values between 18.5 and 24.9 are considered "normal" or "healthy" weight. BMI values between 25 and 29.9 are considered "overweight" and 30 and above are considered "obese." BMIs above 25 are unhealthy and have been shown to increase the risk of certain chronic diseases. BMIs under 18.5 are considered "underweight."