Body fat scales use a method known as bio-electrical impedance (BIA) to estimate your body composition. They run a light electrical current through your body, measure the degree of resistance (or impedance) to the flow of the current, then use this information to estimate how much body fat you have.

Body fat scales are one of the most popular ways to measure body composition, mainly because they’re quick and easy to use. Some devices even connect to your computer and provide numerous graphs, charts and diagrams. But while it might look very scientific and official, much of this information is completely useless.

In one study that looked at changes in body composition in a group of male bodybuilders, researchers compared several body fat tests — including skinfold measurements, dual-energy x-ray absorptiometry (DXA), BIA, and underwater weighing — with something called the 4-compartment (4C) model.

Body fat scales are one of the most popular ways to measure body composition, mainly because they’re quick and easy to use. Some devices even connect to your computer and provide numerous graphs, charts and diagrams. But while it might look very scientific and official, much of this information is completely useless.

BIA was the least accurate of all the methods, with a margin of error as high as 8%. It was even worse than BMI.

BIA had the biggest spread of all the methods, even bigger than BMI. The error rate got as high as 8%, and that’s for detecting a change over time. So one could lose 4% body fat, yet BIA might show an increase in body fat of 4%.