What is BMI?

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Body mass index (BMI) is simply a ratio of height to weight. To calculate your BMI, start with your weight in pounds. Divide that by your height in inches, squared. Then multiply the product by 703. For example, someone who is 6 foot and 170 pounds would have a BMI of 23.

BMIs are categorized as underweight (below 18.5), normal (18.5 " 24.9), overweight (25 " 29.9), and obese (30 and above).

A higher BMI by itself, even in the obese range, may not necessarily indicate a health risk. Take professional athletes, for instance. Their muscle mass, not their fat, tends to bump them up higher on the BMI chart. A case in point: Star running back Ray Rice of the Baltimore Ravens, at 5 foot 8 inches and 212 pounds, fits squarely into the "obese"category according strictly to BMI, despite his lightning speed and superb cardiovascular conditioning.

Some experts prefer to look at different measures of obesity, such as percent body fat, waist to hip ratio, or even just waist size—or perhaps all of these together—which they feel may provide a better picture of health risk.

Nonetheless, BMI is a readily available, easy-to-calculate starting point for both patients and clinicians, and it is widely used in health research to characterize the general population—Super Bowl veterans aside.