Many of us rely on fitness trackers to measure things like our heart rate, step count and sleep quality – but are they really accurate? The answer depends on which health metric you’re looking at. Most of them can give you a good idea of your step count if you’re going on a hike, but even the best fitness trackers struggle to accurately record when you’re sleeping and when you’re awake. 

We combed through the latest studies – and spoke to experts – to find out which metrics you can really trust.

Calories