How Body Fat Scales Work

Muscle weighs more than fat, so as you begin to work out, you might actually gain weight as you start to get in shape. In order to help you see how your diet or workout routines are helping you lose fat, try a scale that measures body fat in addition to weight. Although they are not as accurate as a manual body fat measurement (using calipers), body fat scales might be able to help you track your efforts to reduce your body fat percentage.

Body fat scales might be more fun than functional.

Electronic Measurements 

Body fat scales use bioelectrical impedance analysis (BIA) to measure body fat content. A mild electrical current goes up one of your legs and down the other, measuring the amount of resistance or interference the current encounters on its journey through your body. An electrical current faces less resistance, or impedance, passing through less-dense muscle than it does your more compact fat. The more resistance, the more fat.

The BIA can’t actually measure the percentage of fat in your body, but it can ascertain body density. Once it has done this, a computer in the scale translates your results into an estimated body fat percentage. This percentage is based on your height and age, which must enter into the scale.

Shop for and Compare Body Fat Scales (with reviews)

Questionable Accuracy

Body fat scales don’t provide true, accurate readings of your body fat percentage. However, they can give you accurate readings of increases or decreases in body fat, relative to each other, if you use the scale the same way each time. For example, if your body fat is 19 percent and you’re given a reading of 21 percent by the scale, if you lose 1 percent of body fat and then use the scale again (in the same way you previously used it), your new body fat percentage of 18 should be displayed as 20 percent on the scale.

In order to preserve this relative accuracy, you must use the scale in the same way each time. For example, if you use the scale after being outside in the freezing cold, the results will be different from a reading taken after you’ve just finished a body-temperature raising, 60-minute workout. Because your body temperature is different during different times of the day, even weighing yourself in the evening vs. a mid-morning weigh can skew the results. In addition to differences in weighings on a particular scale, weighings performed on different scales may also result in different displays of body fat percentage.

Consumers Union says that body fat scales are inaccurate. It noted that a 2003 review of them found them consistently inaccurate when testers using the machines were also measured using highly reliable dual-energy X-ray tests.

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