by Andrew Fink

Created on: June 15, 2008

BMI is an acronym for Body Mass Index, one of many different measures used to quantify an individual's overall health. BMI is one of the easiest to compute, given that it works only from an individual's height and weight. The formula is intended for use on adult men and women, and will not give accurate measurements for children, whose bodies are still developing. The formula to calculate BMI is this:

For English measurements:
[([weight in pounds]) / ([height in inches] x [height in inches])] x 703 = BMI

For metric measurements:
([weight in kilograms]) / ([height in meters] x [height in meters]) = BMI

Here are two example BMIs:
Carol weighs 142 pounds and is 5 foot 8 inches tall. After converting her height into inches (5 ft x 12 + 8 in = 68 inches) we calculate her BMI as such:

[(142 lbs) / (68 in) x (68 in)] x 703 = a BMI of 22.1

David weighs 103 kilograms and is 181 centimeters tall. After converting his height to meters (181 cm / 100 = 1.81 m) we calculate his BMI as such.

[(103 kg) / (1.81 m) x (1.81 m)] = a BMI of 31.4

There are also a variety of online calculators online that can be used, such as: http://www.nhlbisupport.com/bmi/

The scale for BMI is thus:
less than 18.5 - underweight
between 18.6 and 24.9 - normal weight
between 25 and 29.9 - overweight
more than 30 - obese

Thus, Carol would have a BMI in the normal weight range, while David would be just crossing into obesity.

While a convenient and easily understood measure of health, BMI calculation does have a few flaws. One of the primary ones is its inability to incorporate body fat percentage into its calculations. What this means, is that even if you have a low body fat (a good indication of overall health), you can gain weight from other factors, such as a thicker bone structure. In these cases, BMI could miscalculate your actual health. This is particularly true for individual's with large builds, because muscle actually weights more than body fat. Thus, if a David's weight was actually gained from bodybuilding, he may not be obese at all, but you would never know from his BMI.

A second flaw is its inability to consider where fat is stored on the body. Recent studies have shown that an individuals waist to hip ratio may be a better indicator for overall health than BMI. If a person has more weight stored in their waist or stomach, it has been shown that they have a much greater risk for diabetes and heart disease than individuals who tend to store fat or their hips and buttocks. For this reason, women with high BMIs can still be healthy, because they tend to store their fat on their hips. Men on the other hand, tend to store fat along the waistline, and can still be at risk for lifestyle related illnesses at lower BMI levels.

While a convenient, and generally accurate, indicator of health, BMI is best used in conjunction with other, more accurate tests, such as body fat percentage and waist to hip ratio. There are many different tests out there, and researching and implementing a variety of them is key to gaining an accurate picture of your overall health.

Learn more about this author, Andrew Fink.
Click here to send this author comments or questions.