BMI, or body mass index, is flawed as a tool for determining body fat. BMI considers only your height and weight; it does not take into consideration body composition. However, BMI is still useful for determining whether you are overweight or obese, although a more accurate measurement tool is available by measuring your percentage of body fat.


BMI is an index that calculates body mass using weight and height. To calculate it, divide your weight in pounds by your height in inches squared, and then multiply the result by 703. The result is your BMI. If it is under 18.5, you are underweight. If it is between 18.5 and 24.9, you have a normal weight. If it is between 25 and 29.9, you are overweight. If it is 30 or above, you are obese. However, these results may not be totally accurate, as your body composition may not fit your BMI category.

Why BMI is Flawed

BMI is flawed because it does not consider your body composition. Take a professional football player as an example. Professional football players have a very large muscle mass, and thus weigh more overall than average people. According to the BMI scale, most football players are obese, even though they often look as if they are pure muscle. BMI does not consider what your body weight is made of, and thus can be inaccurate. BMI cannot differentiate between 200 pounds of body fat and 200 pounds of muscle mass.

A More Accurate Measurement

BMI can be a good starting point to assess your weight status, but combining BMI with body composition is much more accurate. A measurement of body composition can tell you what percentage of your body is fat and what percentage of your body is muscle. If you are classified as obese according to BMI, but then get your body composition measured and find that it is mostly muscle, you are not truly obese. On the other hand, if you are classified as obese according to BMI and have a high fat percentage, you know that the obese classification is correct.

Body Composition Considerations

To get your body composition measured, talk to your doctor or local fitness facility. Skinfold calipers or a bioelectrical impedance scale will most likely be used to measure your body fat percentage. According to the American College of Sports Medicine, for a man aged 20 to 39, a body fat percentage above 19 would be considered overweight. For a man over 40, a body fat percentage over 22 is overweight. For women aged 20 to 39, a body fat percentage over 32 is considered overweight, and for women over 40, a fat percentage over 34 percent is overweight. Use these guidelines in conjunction with BMI to determine your weight status.


  • ACSM’s Guidelines for Exercise Testing and Prescription, seventh edition; Mitchell H. Whaley
  • ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription, fifth edition; Leonard A. Kaminsky

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