Body mass index, or BMI, uses a simple equation to measure the relationship of your height to your weight, telling you if you have a healthy level of body fat. Although body mass index is a good measure for much of the population, it's not infallible. In muscular body types — particularly bodybuilders or other stocky, beefy athletes — your BMI may show you as being overly fat even though you're lean and healthy. You can't adjust the BMI equation to account for an athletic build. Instead, rely on other methods to determine if you're at a healthy weight.
If you have an athletic build, the classic body mass index or BMI calculation may not give accurate results. You can use other methods, including waist circumference, blood tests and tests of body composition, to monitor your health.
BMI Concerns for Athletes
According to the Centers for Disease Control and Prevention, a normal BMI is between 18.5 and 24.9. If yours measures between 25 and 29.9, you're classified as overweight, and if it measures over 30, you're considered obese. This classification applies to the general population, in which a heavier weight is likely due to an abundance of body fat, not muscle. Too much body fat raises your risk of chronic disease, including heart disease, type 2 diabetes and hypertension.
Muscle is a denser tissue than fat, and an abundance of lean mass raises weight on the scale — which means you'll have a high BMI. However, because your excess weight comes from muscle, not excess fat, you won't have the related health risks associated with carrying too much body fat.
Read more: The Ideal Weight for an Athletic Build
Body Composition to Assess Health
Athletes have a body fat percentage that's lower than that of the general population, though female athletes still naturally carry more body fat than men because of the needs of childbearing. A normal body fat percentage for women is between 25 and 31 percent. For men, it's between 18 and 24 percent. For athletes, body fat percentage usually runs 14 to 20 percent for women and 6 to 13 percent for men. If you're female and your body fat runs 32 percent or higher or, for men, 25 percent or higher, even if you're athletic, you may be overly fat and have an increased risk of health complications.
Estimate your body fat using a specialized scale or have a fitness professional use body calipers for the measurement. More precise methods, such as underwater weighing and DEXA scans, are available, but they require specialized equipment and trained professionals.
Read more: BMI Vs. Body Fat Percentage
Waist Circumference as Fitness Measure
Not all body fat is created equal. The type that gathers inside your abdominal wall and around your internal organs is far more dangerous and inflammatory than the fat that sits just under the skin at your hips, thighs, upper arms and belly.
A waist circumference greater than 40 inches around, if you're male, or, if you're female, 35 inches around, indicates an abundance of harmful deep-abdominal fat and a higher risk of chronic disease. A big, muscular frame doesn't exclude you from carrying too much belly fat, so use a tape measure to find your own waist size, at the point just above your hip bones, to ensure you're within a healthy range.
Blood Tests to Measure Health
Body weight and fatness is not the only risk factor for chronic disease and metabolic disorders. Although it's unlikely, athletes can look healthy on the outside and take impeccable care of their bodies, and still be at risk. If heart disease or metabolic disorders run in your family, your doctor may run blood tests to check your blood pressure, glucose tolerance, blood lipids and cholesterol levels to give you a clear bill of health. These tests help you confirm the health of your body at your weight and body composition.