Are bodyfat scales accurate?

Not for a single morning. In a lab test of 15 devices in 73 adults, home scales missed the true bodyfat by anywhere from about 3.5 points too low to about 12 points too high on average, depending on the model. They were better at tracking change: over 12 to 16 weeks, the typical error on the change was about 2 to 3 points.

Why they miss

A bodyfat scale sends a small current up through your feet and estimates your fat from how easily it passes. Water carries the current, so what you drank, ate and sweated can nudge the number. Each brand also uses its own formula, which is why two scales can disagree by several points on the same morning.

The weight itself is accurate. In a 2021 study of three smart scales against a DEXA scan, the weight was off by well under a pound, while the fat mass was off by a median of about 5 to 10 pounds (2 to 4 kg).

A bodyfat scale is like a clock that runs fast. You can’t trust the time it shows, but you can roughly trust how much time has passed.

How to make it useful

• Same conditions every time: first thing in the morning, after the bathroom, before food or drink.
• Use the weekly average, not the daily number.
• Compare months, not days. A change smaller than about 2 to 3 points over a few months may be noise.
• Never compare brands, or your old scale with a new one.
• Check it against your waist. If your weight and waist are falling, you’re losing fat.

Gyroscope logs your weight and bodyfat from a Withings or any Apple Health scale on its own, free on any plan, so the trend builds without typing.