An interview with Ed Kippel, senior program leader, Personal Care and Wellness, Consumer Reports.

Q: Why are people interested in body-fat scales?
A. Concerns about fat and obesity have spawned the body-fat bathroom scale. Besides giving your weight, it measures the percentage of fat on your frame.

Q: Why doesn't Consumer Reports test them?
A: We did, back in 2003. We found that none of the scales were accurate for all of our subjects. We compared them with results from a dual-energy X-ray test, or DEXA, which is one of the gold standards for body-fat measurements. There was quite a variability in how they performed.

Q: But hasn't technology improved since then?
A: No, which is one reason we're reluctant to test the scales again. The scales still calculate body-fat percentage by means of a weak electrical current, which sends a signal up one leg, across your pelvis, and down the other leg. Many things can throw off the reading, such as your distribution of body fat, whether or not you have a knee or hip implant, even whether your feet are clean.

Q: Isn't it worth knowing your body-fat percentage even if it's not quite accurate?
A: Not really. There's little reliable data relating ranges of body-fat percentage directly to health outcomes. Most of the information that's available relates health to the Body Mass Index, which does not directly measure body-fat percentage but is a more accurate indicator of overweight and obesity than weight alone. To find your BMI, you don't need to buy a body-fat scale. All you need to know is your height and your weight, and you can calculate your BMI easily on our Web site. Two other ways to gauge your personal risk—methods that could be more accurate—are to measure your waist circumference or your waist-to-hip ratio. Your waist circumference should be less than 35 inches around if you're a woman, and less than 40 inches if you're a man. For the waist-to-hip ratio, divide the circumference at the narrowest part of the waist by the circumference at the broadest part of the hip. A healthy waist-to-hip ratio is 0.8 or less for women and 0.9 or less for men.