How Does a Scale Measure Body Fat?

While you can hardly turn on the television or open a magazine without reading about the dangers of obesity, your body fat percentage -- the amount of your weight that comes from fat vs. lean mass -- is actually a better look at your disease risk. Some at-home scales can estimate your body fat using painless electrical signals -- a technology called bioelectrical impedance analysis, or BIA. At-home body fat scales aren't all that accurate, however, so you should consult a professional for a precise reading.

Scales are good at measuring weight but not so great at measuring body fat. (Image: Christoph Wilhelm/DigitalVision/Getty Images)

The Principle Behind Body Fat Scales

A BIA scale sends a painless electrical signal through your body. It uses the time this signal takes to travel from the starting point to the ending point -- usually up one leg, across your pelvis and down the other leg -- to determine about how much fat you have. The electrical signal passes more quickly through water -- or muscle, which contains a high percentage of water -- than through fat, so the longer the signal takes, the higher your body fat percentage.

Types of Body Fat Scales

Most scales that measure body fat simply place electrodes under the foot panels of the scale. For a more accurate analysis of your body fat, you may want to find one of the BIA devices that has hand electrodes as well. These hand-to-foot BIA measurements give more accurate readings, notes a review article published in Nutrition Journal in 2008. Foot-to-foot measurements can underestimate body fat because they don't travel across the stomach like hand-to-foot BIA measurements, and this is where many people carry their extra weight.

Accuracy at Measuring Body Fat

When Consumer Reports tested a number of bathroom scales that measure body fat in 2016, they found that while most were accurate for weight, none of them was particularly accurate in predicting body fat. Being dehydrated or overhydrated, having very dirty or calloused feet or carrying most of your weight in your belly could all make a scale using BIA less accurate.

These scales also may be more likely to overestimate fat in lean people and underestimate it in very heavy people. They aren't as accurate in children, the elderly, highly trained athletes or people with osteoporosis. Certain medical conditions -- including muscular dystrophy, polio, cirrhosis, amputations and congestive heart failure -- interfere with the scale's accuracy.

People of varying population groups often have differences in average arm and leg lengths and body fat distribution. As a result, the scales would need to use separate equations to properly predict body fat in each population group, which is another potential reason for the scales' inaccuracy. You'll get the best and most consistent results if you use the scale at the same time of day, and avoid using it right after exercise or immediately after eating or drinking.

More Accurate Methods

Body fat scales can give you an idea of how body fat levels change from week to week, but other methods provide more accurate measurements. Skinfold measurements taken by a trained professional using calipers can be a relatively inexpensive and accurate way to get an idea of your body fat. Some health clubs offer this service.

Other, more expensive measurements can give you accurate results, but you'll need to visit a clinic. These include underwater weighing and various types of full body scans, such as CT scans, MRI scans and DEXA scans.