According to the Centers for Disease Control and Prevention, body mass index is calculated by using weight and height. If the measurement system used is the International System of Units, the weight of the individual in kilograms is divided by the height in meters squared. If the measurement system used is the United States Customary Units, the weight in pounds is divided by height in inches squared multiplied by 703.
The results of both of these calculations gives a number that is within four different weight ranges - underweight, normal, overweight, and obese. The Center for Disease Control and Prevention claims that these calculations for body mass index are fairly accurate in illustrating the correlation between body mass number and body fatness.
However, the CDC states that these calculations do exhibit some discrepancies. For example, women have more body fat than men, but the body mass index between those men and women are identical. Also, the elderly tend to have greater body fat but not a different body mass index in comparison to younger adults. Professional athletes who have a high muscle content generally store more weight, which causes their body mass indexes to be excessively high in relation to their heights. In this case, body mass index is not a good indicator of physical fitness.