The formula for calculating body mass index, or BMI, is weight in pounds divided by height in inches squared and then multiplied by 703, according to the CDC. This formula is identical for men, women and children.
Although BMI is commonly used as a screening tool for obesity, the CDC cautions against using it as a diagnostic tool, as it doesn't differentiate between fat mass and muscle mass. BMI has also been criticized because it doesn't take into account differences in body type and where fat is stored, explains WebMD. Because of this, the CDC suggests measuring the circumference of the waist to determine whether or not an individual is likely to develop obesity-related illnesses and to quantify overall health.
WebMD also points out that the BMI doesn't take into account differences due to race, age and gender. For example, the risk of developing weight-related issues begins at a lower BMI for Asians than it does for Caucasians. Asians are also labeled obese at a lower number than other races. Furthermore, women generally have more body fat than men even when they have the same BMI.
The CDC states that any BMI between 18.5 and 24.9 is considered healthy. A BMI of 30 or above is considered obese.