Millions of people wear some kind of fitness tracking device. But just how accurate are they?

They're accurate enough to be useful for most people for most purposes. But it also varies by what it is you’re trying to measure.

Asst. Prof. Lisa Cadmus-Bertram University of Wisconsin-Madison The shallow end of fitness tracking is a simple step count.

It's better if the tracker is worn on the hip, not the wrist. Devices that measure heart rate tend are good at measuring heart rate at rest but have much more variability during exercise. And a recent meta-analysis showed that fitness trackers are not very accurate at measuring caloric burn.