3 Ways to Tell Whether You're Fat (It's Not as Simple as You'd Think)

Body mass index has pros and cons; waist circumference and waist-to-hip ratio are other options.

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You'd think it would be easy to judge whether you are carrying too much body fat. Not so. Plenty of us, especially women, think we're fat when actually we're just not as skinny as the actresses on Gossip Girl. And on the other side of things, nearly half of people who are obese don't think their weight is a problem, according to obesity expert Barry Popkin.

The current issue of the Harvard Health Letter offers three low-tech ways to measure fatness. You've probably heard about body mass index, or BMI, which is the simplest way to go: Just input your height and weight into one of many Web calculators or charts, and you'll get back a number that pegs you as underweight, healthy, overweight, obese, or extremely obese.

You may also have heard about the criticism of BMI. For one thing, people who are really muscular, like pro athletes, may have a BMI score in the "overweight" range when they actually have very little body fat. "That deficiency is true but unusual," says Anthony Komaroff, editor of the HHL. (Few of us, after all, are as muscular as some pro athletes.) More worrisome is the idea that BMI might give a misleading impression to people who are normal weight but have a high percentage of body fat and therefore a higher risk of heart disease, diabetes, and hypertension. That can be a problem for older people, who are losing muscle and thus gaining body fat percentage points, says Komaroff.

A method that includes a little more effort is measuring waist circumference; this hinges on the idea that the fat that accumulates around the waist is what really matters in terms of disease risk. The HHL article says it's not even clear where on the waist to measure, but Komaroff says that for most purposes, wrapping a measuring tape around the upper hipbone, which should run across your navel, will do. The guidelines here: If you're a man with a waist larger than 40 inches or a woman with a waist larger than 35 inches, you need to lose some fat. (Those may need to be revised even lower, the article says, so don't be complacent if you're a man measuring in at 39.5 inches.)

Finally, the article examines the waist-to-hip ratio. To calculate it, measure your waist at its narrowest point and divide that number by your hip circumference, as measured at its widest point. (A woman with a 29-inch waist and 37-inch hips, for example, would have a WHR of 0.78.) The general guidelines are no higher than 0.85 for women and 0.9 for men.

In addition to these three approaches, you may have heard of high-tech fat measurement devices. In fact, U.S. News's Sarah Baldauf offered herself up as a guinea pig to test some in 2007. They're useful for research, says Komaroff, but the average person doesn't need to seek them out. The simpler and cheaper methods discussed above can give you a pretty accurate gauge of whether you need to keep a better eye on your diet and exercise routine.