Body mass index, or BMI, is calculated by dividing weight by height squared and then multiplying by 703, the Centers for Disease Control states. The BMI provides an inexpensive and easy method of determining body fat and is used as a screening tool for potential health problems related to weight.
Although the formula for children is the same as the adult formula, a child's BMI should take into account that girls and boys have different body fat levels that also change as they age, according to the CDC.
The CDC clarifies that BMI is not a diagnostic tool because it doesn't account for muscle mass in its calculations. Although BMI generally has a strong direct correlation with body fat, the correlation is not perfect; age, sex, race and activity level all affect BMI.
When determining whether an individual is at risk for obesity-related disease, BMI should be taken into account along with waist circumference and other risk factors such as genetics and fitness, the CDC states. Athletes with a large amount of muscle technically are considered overweight or obese under BMI guidelines despite having a low body fat percentage, but they are obviously not subject to the same health concerns as those with a high body fat percentage.
According to the CDC, a healthy BMI ranges from 18.5 to 24.9. A result below 18.5 is underweight. A result greater than 24.9 is overweight, while a number greater than 30 is considered obese. By itself, the BMI is not a sufficient diagnostic tool and does not accurately evaluate health. The CDC recommends looking at waist circumference, activity level, eating habits and other risk factors to create a more accurate health profile.