Step right up to a body-fat scale

Enter the brave new world of smart scales, which manufacturers claim will measure not just your overall weight but also the percentage of your weight that comes from fat vs. muscle, bone and water. Some of the scales also calculate your body mass index, or BMI, which measures your weight in relation to your height.

Consumer Reports recently put body-fat scales that come with a ton of high-tech claims through their paces.

The Body-Fat Bust

Consumer Reports looked at six digital scales that gave readings for weight and body fat (some calculate BMI as well), ranging in price from $40 to $150.

Testers checked first to see how well they performed the most basic function: giving users an accurate weight. To do that, testers had seven men and eight women step on an electronic lab scale, which was used as a control, then weighed them four times over two days on each of the scales. Five of the scales read weight accurately.

Rating the scales on their ability to gauge body fat was more complicated, and the results were unimpressive: None was very accurate. Consumer Reports knows that because its testers got benchmark readings by using a measuring tool called a Bod Pod at Sacred Heart University in Fairfield, Connecticut.

Six volunteers – three men and three women, some heavier, some slimmer – climbed into the Bod Pod. The egg-shaped device determines body fat by precisely recording the change in air pressure created when a body is sealed into a closed chamber.

The scales that were tested measure body fat using an electric current that travels through your body when you step barefoot onto the device’s metal footpads. That tiny current — much too low to feel, or to cause any harm — passes up one leg, through your pelvis and down your other leg.

The resistance the current encounters as it travels depends on the proportion of fat and muscle tissue. Based on that, the scales use built-in formulas to estimate the percentage of your weight that comes from fat.

For some of the test subjects, the scales overstated their body fat; for others, the scales understated it. The scale that came closest to the Bod Pod results was still off by up to 21 percent; the worst performer was off by up to 34 percent.

Back to Basics

If high-tech scales aren’t very good at determining your body fat, what’s a dieter to do? You could go to a lab with a Bod Pod for $75 per session. Or

you could just use a measuring tape and a calculator.

To calculate your BMI, divide your weight in pounds by your height in inches squared; then multiply by 703. For example, a 140-pound, 5-foot-6-inch person has a BMI of 22.6 (140 divided by 66, divided by 66, times 703).

Aim for a BMI of between 18.5 and 24.9. For men and women, a BMI of 25 or more is considered overweight, and a BMI of 30 or higher is considered obese. You should also watch your waist size because belly fat, more than fat elsewhere on your body, is linked to an increased risk of cancer, heart disease, type 2 di- abetes and early death. In women, a waist of 35 inches or greater is cause for concern; in men, it’s 40 inches and up. The best way to measure your waist is a simple measuring tape. Just wrap it around your bare abdomen at about the level of your belly button, then measure after exhaling.

IMAGE BY CONSUMER REPORTS The smart scales that Consumer Reports tested can’t hold a candle to the accuracy of a Bod Pod, a scientific instrument used to gauge body fat.