To find your BMI, multiply your weight in pounds by 703, take your height in inches and square it, then divide the former by the latter. Websites, such as the site for the National Heart, Lung and Blood Institute, offer BMI calculators as of March 2015.
A BMI of less than 18.5 is considered underweight, between 18.5 and 24.9 is considered normal, 25 to 29.9 is considered overweight and 30 or higher is considered obese. Since BMI is only a ratio between height and weight, it does not always accurately assess a person's overall health and fitness. Those with large amounts of muscle mass may be healthy, but the BMI standards could classify them as overweight or obese.
Age is not factored into the BMI formula, but the CDC uses a BMI percentile chart for children and teenagers to determine if they are in a normal weight range. Since children have not physically matured, their BMI's are not comparable with those of adults. Due to the limitations of the BMI formula, a family physician can use additional methods to determine if a high BMI is a cause for concern. These methods may include skin fold measurements, reviewing family medical history, and asking about the child's diet and activity level.