How accurate are the calorie numbers on fitness trackers?
Considerably less accurate than the confident display suggests — heart rate is measured reasonably well; calories are estimated badly, and the two get conflated because they appear on the same screen.
What is actually measured. A wrist tracker measures heart rate optically, by shining light into the skin and detecting blood volume changes, plus motion from accelerometers. Everything else — calories, sleep stages, stress, recovery — is inferred from those signals through proprietary models.
How accurate each is:
Step counting is generally good on walking, less so pushing a trolley or holding handles.
Heart rate is good at rest and steady effort, and less reliable during rapid changes, high intensity, cold weather, and with wrist movement — chest straps measuring electrical activity remain substantially more accurate for interval work.
Energy expenditure is the weak point. Validation studies have repeatedly found errors commonly in the range of 20–30%, and sometimes far higher, varying by device, activity and individual. Errors are not random: steady cycling and weight training are typically estimated worst.
Why estimating calories is hard. Energy expenditure depends on body composition, efficiency, fitness, temperature and the activity's mechanics — none of which a wrist sensor can know. The device applies a population model to an individual.
Why it matters practically. People eat back exercise calories the device reports, which — if the figure is inflated by a quarter — quietly eliminates a deficit. Treating tracker calories as budget is the commonest way a tracker undermines the goal it was bought for.
What trackers are genuinely good for: consistency and trends over time, since a consistent error still shows whether you did more than last week; resting heart rate, which is a meaningful health signal; activity prompts, which demonstrably increase movement; and adherence, through the behavioural effect of measurement itself.
How to use them sensibly: compare against yourself rather than against other people or other devices; ignore the absolute calorie figure; use heart rate zones as rough guides, not gospel; and treat sleep-stage breakdowns as approximations.
General information, not medical advice.