21st Oct 2009

Understanding Your Body Mass Index

Body Mass Index (BMI) is a widely used indicator for health and fitness. A person’s Body Mass Index is calculated by dividing a person’s weight (measured in kilograms) by their height (measured in meters) squared. Mathematically, BMI looks like this: BMI = weight (kg)/ height (m)^2. That formula uses the internationally accepted metric system. If you want to find your BMI using the American standard pounds and inches, you would use this formula instead: BMI = weight (pounds) /height (in)^2 x 703. You can see that in the formula that uses pounds and inches for measurement units, you multiple by 703 the result of dividing your weight by the square of your height.

So what good does it do to calculate what may seem like a random number, and call it your Body Mass Index? Health experts have created a chart that makes a general classification that correlates your height with your weight. Using the figure from the BMI calculation, you can determine whether, according to the inventors of the corresponding weight status chart, you are underweight, normal, overweight, or obese. Here’s how the chart segments those weight statuses based upon your BMI.

BMI Weight Status

  • < 18.5 Underweight
  • 18.5 – 24.9 Normal
  • 25.0 – 29.9 Overweight
  • 30.0 and above Obese

The main problem with this simplified classification system involves the measurement of BMI in athletes. For instance, if you went to your local gym and performed BMI calculations, you’d find a lot of guys who definitely don’t look overweight, but whose Body Mass Index would fall somewhere above the 25.0. Are these people really overweight? You’re likely to find NFL running backs who have a body fat percentage of less than 12% who would be classified as overweight by this standard.

I consider myself to be athletic. I’m 5’9”, and I weigh 188 pounds. My BMI is 27.8. That leaves me in the overweight group. The last time I tested my body fat percentage, it was 15.5%. Although I admit that I could shed a few pounds to be in prime condition, I wouldn’t say that I’m overweight.

The Body Mass Index concept was invented in the 1800s, and has been used since then by researchers to study populations. In studying BMI calculations in large numbers, an overall picture of increases, decreases, or simply current population standing as it relates to fatness can be drawn. Athletes and others who build lean muscle are outliers in these wider studies. They need have their self esteems affected because they fall into an above average fat category.

To get a better feel for how fat or fit you are, there are other indicators that you can use. Getting an accurate body fat reading using a body fat caliper or an electronic body fat analyzer device gives a much clearer understanding of where you stand as it regards your health.