What Does BMI Really Tell You?

As a barometer of obesity, many Americans use the Body Mass Index (BMI). This is a formula that adjusts a person’s weight for their height and calculates a result that categorizes them as underweight, normal, overweight or obese.

For adults, BMI = (weight in pounds/height in inches squared) x 703. That means someone who is 6-foot-3 (75 inches tall) and weighs 220 lbs. has a BMI of (220/752) x 703, or 27.5.

BMIs are then categorized thusly:

• Under 18.5—Underweight
• 18.5–24.9—Normal
• 25–29.9—Overweight
• 30 or more—Obese

Is this a reliable indicator? To a degree. Generally, BMI correlates to body fat. The higher it is, the fatter you are, and the greater your risk for high blood pressure, cardiovascular disease, diabetes, osteoarthritis and certain cancers.

But there are qualifications. Two people with the same BMI don’t necessarily have the same percentage of body fat. Women are likely to have a higher percentage of body fat than men with the same BMI, and older adults may have more than younger ones. And because weight is only one factor among many related to disease—others include diet, physical activity, blood pressure, blood sugar and cholesterol levels and family history of disease—BMI is not an absolute indicator of anything.

Nonetheless, you can draw some conclusions from it, according to the Centers for Disease Control and Prevention. If you’re in one of the top two categories (obese or overweight), you probably don’t want to gain any more. If you’re obese, or overweight with other risk factors, you are advised to lose weight. As with any change in diet or health practices, consult a physician.

For more on BMI and links to further information about weight, nutrition and disease risk, see www.cdc.gov/nccdphp/dnpa/bmi/.