BMI, or Body Mass Index, is a measure of a person's body fat, based on a simple calculation of body weight in kilograms, divided by the square of the individual's height in meters.

It is used by health professionals around the world to determine whether a person is overweight or clinically obese. A person with a BMI equal to or over 25 is considered overweight, while an index of 30 or more is considered obese. At the opposite end of the scale, a BMI of less than 18.5 is an indication that a person is underweight. The optimum, healthy range is between 18.5 and 25.

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Because BMI doesn't take any factors other than height and weight into consideration, many people argue that it is far too simplistic. A frequently quoted example of where it is likely to be misleading is with regard to muscle mass

Since muscle weighs more than fat, people who have a higher percentage of body muscle, such as athletes, could, on a strict interpretation of their body mass index, be considered obese a classification that for most professional athletes would clearly not be the case.