The body mass index, or BMI, is a measure that estimates body fatness. It is designed as a tool for the general public, however, and not specifically for elite athletes such as bodybuilders. As such, bodybuilders may have a very high BMI but low body fat levels. Alternative measurements are available for bodybuilders who are concerned about their amount of body fat.
About BMI
BMI is a measure of your height relative to your weight. BMI does not measure body fat directly, but in general BMI corresponds with levels of fatness. Because of this, BMI is used to indicate whether someone is a healthy weight or not. A BMI in the range of 18.5 to 24.9 is considered healthy. Less than 18.5 is considered underweight and anything in the range of 25 to 29.9 is considered overweight -- anything greater than 29.9 is considered obese. These are only estimations however, and a medical practitioner may need to perform additional tests to assess a person's weight.
Calculating BMI
BMI is calculated the same for everyone, including bodybuilders. To calculate your BMI, simply multiply your weight in pounds by 703 and divide the product by your height in inches. Then divide the quotient by your height in inches again to get your BMI. For example, if you are 70-inches tall and weigh 250 pounds you would multiply 250 by 703 to get 175,750. You would then divide this number by 70 to get 2,510. Finally you would divide 2,510 by 70 to get a BMI of 35.86 -- which would possibly indicate obesity.
Problem for Bodybuilders
Bodybuilders present a problem for the BMI. Because bodybuilders have massive muscle weight, this pushes their BMI up even if they have little fat. For example, when Arnold Schwarzenegger was a bodybuilder, his competition weight was approximately 235 pounds. At 6-feet, 2-inches tall, this would give him a BMI of 35.2 -- which would erroneously classify the eight-time Mr. Olympia champion as obese.
Altenatives
BMI is a simple measure that costs nothing. But if you are concerned about your fat levels as a bodybuilder, you will need to rely on a method that measures fat directly. Some methods include measuring skin folds with calipers, underwater weighing, bioelectrical impedence, dual-energy X-ray absorptiometry and isotope dilution. Talk to your doctor about these options if you feel that your fat levels may be unhealthy in spite of your muscle.