The Body Mass Index (BMI) is a calculation based on your height and weight used to determine how at risk you are for many obesity-related health problems. However, because BMI only takes height and weight into account and not body fat percentage, it's often a flawed form of assessing your health risks.
How BMI Works
Your BMI number is derived from this formula:
(weight in pounds x 703) / (height in inches x height in inches) = BMI.
Based on the number you get from that formula, you're placed into a category:
Below 18.5 Underweight
18.5 -24.9 Normal
25 - 29.9 Overweight
30 & Above Obese
If you are an average man in your 30's, chances are you weigh about 180 lbs and are 5'10" tall. Putting your numbers into the formula would look like this:
(180 x 703) / (70x 70) = 25.8
This puts you in the overweight category.
Pros For BMI
BMI is used universally. The formula requires little information and is easy to calculate.
For the very obese, where any other test is difficult, BMI is the simplest option for a health test.
Because BMI is one number, it provides a definite goal.
Major Flaw
BMI doesn't take body fat into account. If you have two people with the same height and weight as above but one is 13 percent body fat and the other is 30 percent, there is a definite difference in health risk.
Other Flaws
Bone density is another factor that can affect weight. The higher your bone density, the higher your weight and the BMI.
Pregnant women also gain weight, which is healthy and a necessity. This gain in weight will also increase the BMI.
While pregnancy and a higher bone density increase BMI, they do not increase obesity-related health risks.
Body Fat Percentage a Better Assesment of Risk
It's better to asses obesity health risk factors based on body fat percentage and not height and weight.
Body fat can be tested in a number of ways that can be just as easy as BMI, but far more accurate to your health risks. Calipers are used to pinch a few sites on your body to measure subcutaneous fat. Bioelectrical impedance sends a very low voltage electrical current through your body. Body fat is then measured based on the speed at which the electrical current passes through the different tissue in the body
References
- Third Edition: Measurement by the Physical Educator: Why and How, David K. Miller, 1998
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