Some of today's bathroom scales measure not only your overall weight but also the percentage of your weight that comes from fat vs. muscle, bone, and water.

Known as body-fat scales, some also calculate your body mass index, or BMI, which measures your weight in relation to your height. And still others allow you to upload all of that information to your smartphone or computer, so you can track changes in your weight and other measurements over time, and if you choose, share your progress with your workout buddies.  

We took note of which models had which features, talked with weight-loss experts to learn how important it is to have those extras, then did some tests to find out how well the main features worked.

The Body-Fat Bust

We 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.

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

Five of the body-fat scales read weight accurately, but one, the Qardio QardioBase (which happened to be one of the priciest), routinely overestimated weight by 6 to 10 pounds. When we asked Qardio to respond to our findings, the company told us that it was aware of the problem and said it was corrected. Consumer Reports investigated Qardio's fix; here's what we found. Customers can ask for a refund or replacement at support@getqardio.com. (For more details on all six scales, check our Ratings and buying guide.)

Rating a body-fat scale on its ability to gauge body fat was more complicated, and the results were unimpressive: None was very accurate. We know that because we got benchmark readings by using a measuring tool called a Bod Pod at Sacred Heart University in Fairfield, Conn.

A woman sits in a BodPod as a tester prepares to measure her body fat. It is more accurate than a body-fat scale.
The body-fat scales we tested were nowhere near as accurate as a Bod Pod, an instrument used to gauge the body fat of our volunteers.

How We Tested

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. (Research shows that method is as accurate as the other gold standard—measuring how much water a body displaces when dunked into a tank full of water—but the Bod Pod measure is quicker and easier to perform.)

The scales we 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 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 our test subjects, the scales overstated their body fat; for others, the scales understated it. The body-fat 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. At least they were consistent, though, so you can use the scales that scored better at body-fat repeatability to track relative gains and losses over time. 

How a device errs may depend in part on body shape. Because the electric current passes through only your legs, the scales might underestimate body fat if you have a big belly but slimmer legs and overestimate it if you carry more weight on your hips and thighs.

Results can also vary depending on whether your feet are wet or dry (they should be dry, according to the instructions), whether you recently exercised, and how much water you had to drink. And the devices don’t work well if you have an artificial hip or knee because the man-made materials can interfere with the electric current.

Back to Basics

If a smart scale isn'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.

“Take two measurements: your body-mass index and your waist measurement,” says Steven Heymsfield, M.D., a professor of metabolism and body composition at the Pennington Biomedical Research Center in Baton Rouge, La., and an expert on weight loss.

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).

Certain BMI scales we looked at will do that calculation for you. Another option: Use a basic weight-only scale (the Taylor 7506, $25, got top scores in our previous tests of stand-alone digital scales) plus this BMI calculator.

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 diabetes, 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.

Privacy Precaution

A smart scale is designed so that you can upload your weight and other health info to your computer or an app on your phone. But unless you opt to share your results, your numbers are not visible to other users. Whether the body-fat scale manufacturer or software provider has access to that information is another question.

To find out what data are collected and how that information might be used, read the permissions section in the app’s description and the privacy policy on the manufacturer’s website.

Are Fancy Features Worth Paying For?

FeaturesOur Take
Multiple modes. Some body-fat scales have different modes that give you target numbers.

Skip the athlete mode. Unless you are training for the Olympics, that mode will measure your body-fat percentage as less than it really is.

Bone- and water-weight readings. Some scales are claimed to measure bone and water weight.

We did not test those features because we had no way to safely and easily determine their accuracy. To measure bone mass, for example, we would have had to expose volunteers to low-dose X-rays. In addition, the experts we consulted said bone and water weight are not important components of weight for most.

Bluetooth or Wi-Fi connectivity. Part of the appeal of smart scales is that they can send your measurements to your computer or smartphone.Want to use Wi-Fi to connect your scale? Use your phone to check signal strength in your bathroom before you buy the scale.
Multiuser memory. The scales we tested support two to eight users.Choose a scale that stores profiles for the number of people who will be using it.
Ease of use. Three of the scales turned on automatically and recognized our testers. One was extra-easy to read.Features are nice, but accuracy is job one.

Editor's Note: This article also appeared in the March 2016 issue of Consumer Reports magazine.