BMI, or body mass index, is a weight measuring system that compares your body weight in relation to your height. BMI results don't measure your level of physical fitness; rather, they are used to estimate your level of body fat and help doctors determine if your fat levels present a risk for the development of serious health problems.
Understanding BMI
A BMI test does not measure your body fat directly, according to the Centers for Disease Control and Prevention. Rather, it measures your fat indirectly, and serves as a reasonably accurate and less expensive alternative to direct measurement methods such as dual energy X-ray absorptiometry and underwater weighing. During the first part of a BMI measurement, your doctor will take your weight and divide it by the square of your height; she will then multiply this result by 703. For example if you are 65 inches tall and weigh 150 lbs., your BMI is [150 ÷ (65)2] x 703 = 24.96.
BMI Results
If you are 20 or older, BMI results ranging from 18.5 to 24.9 fall within normal body fat levels, the CDC reports. If you have a BMI in a range between 25.0 and 29.9, your body fat levels indicate that you are likely overweight. If you have a BMI of 30.0 or higher, your body fat levels indicate the probable presence of clinical obesity. If you have a BMI below 18.5, you probably have abnormally low levels of body fat. BMI standards are the same for both men and women, while standards for children and teens depend specifically on the age and sex of the child undergoing testing.
Fitness and BMI Accuracy
While BMI is a fairly solid indicator of body fat levels, its accuracy decreases somewhat in certain circumstances, the CDC notes. Potential factors in a relatively inaccurate measurement include the tendency of women to carry more body fat than men at the same BMI and the tendency of older people to have more body fat than younger people at the same BMI. If you are an athlete or otherwise have a high level of physical fitness, you may also appear overweight or obese on a BMI test as a result of unusually high muscle mass.
Alternative Testing
If you are physically fit, you can overcome the inaccuracies of a BMI test with another procedure called a waist circumference measurement, according to the Calorie Control Council. This calculation, which involves measuring your waist with a tape placed at your upper hip bone, focuses exclusively on abdominal fat and tends to minimize discrepancies related to exceptional muscle development. Men with waists in excess of 40 inches typically have elevated health risks, while women with waists in excess of 35 inches have similar risks.
Considerations
A BMI test is not meant to diagnose any specific condition, the CDC explains. Rather, it allows doctors to identify potential obesity-related risks and perform further testing as needed. Potential follow-up procedures include diet evaluations and direct skinfold measurements. Consult your doctor for more information on BMI and its importance to your overall health.
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