Is BMI an Effective Measure of Fitness?

The BMI -- short for body mass index -- is a standard starting point to determine your risk of developing chronic health conditions. It’s a calculation of your height and weight to see if your body composition is on target for your frame size. However, it doesn’t always work for everyone. The chart doesn't account for gender or muscle mass, and can be an inaccurate measurement of fitness level.

Doing the Math

• Your BMI should be between 18.5 and 24.9, according to the Centers for Disease Control and Prevention. If it's below that range, you could be underweight, while if it's greater, you might be overweight. Calculate your body mass index by dividing your weight in pounds by your height in inches squared, then multiply the result by 703. So if you are 160 pounds and stand 68 inches tall, divide 160 by 4,624 -- or 68 squared. Multiply the answer -- 0.034 -- by 703. Your BMI is 24.3, which is in the healthy range.

Fat Locales

• Just because your index falls between 18.5 and 24.9 doesn’t mean your weight is on target. The BMI doesn’t use waist or hip measurement, so it doesn’t factor in where fat is located on your body. Even though your body mass index could be normal, you might have a fat buildup located around your belly -- surrounding your vital organs. Depending on where your fat is located, your risk of developing a chronic ailment could be as great as someone with a BMI in the overweight range of 25.0 to 29.9, or in the obese category of 30 and over.

Muscle Mass

• Training vigorously with weights on a regular basis means you probably have more muscle mass than someone who is sedentary. Muscle weighs more than fat, and the body mass calculation doesn’t account for that. You could be trim, toned and have a low percentage of body fat, but your BMI might be above 25.0 -- a score that puts you in the overweight range, even though you’re clearly very fit. If you have a lot of muscle mass, your BMI could even be in the obese category 30 and aboce, but you're not necessarily obese.

Men vs. Women

• It’s natural for women to have a higher percentage of body fat than men; however, the body mass index doesn’t have a numerical cofactor for gender. Your BMI is the exact same as someone of the opposite gender. If you’re female, you may have more body fat than you should, but your physician could overlook your risk factors for chronic diseases because your BMI seems normal.

Age Considerations

• While the calculation is the same for everyone over age 2, certain age groups have a different reading. Young adults under age 20, for example, need to follow a growth chart, instead of the established values for adults. A healthy BMI of 24.0 for a 25-year-old may not be ideal for someone who is 19. You’ll have to look at the growth charts for your age and gender to see if your BMI falls into the normal range among your peers. The BMI isn’t a strong indicator of healthy weight when you reach your golden years either. Since you have more body fat when you’re older, a BMI that was normal when you were younger might not be ideal later on in life. You could be holding on to too much -- or too little -- body fat, even though the calculation results don’t show it.