Fitness trackers lie on calories
New reporting warns not to trust fitness trackers for calorie‑burn estimates — devices like Garmin and Apple Watch are solid for heart rate and splits, but their calorie calculations can be meaningfully off, so don’t base nutrition solely on those numbers calorie accuracy warning.
A 2017 Stanford study of 60 volunteers tested seven wrist devices — Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2 — and reported heart‑rate errors under 5% for six devices while energy‑expenditure errors averaged 27% and reached 93% on the worst tracker med.stanford.edu. A 2025 University of Mississippi meta‑analysis of 56 studies put Apple Watch mean absolute percent errors at 4.43% for heart rate, 8.17% for step counts and 27.96% for energy expenditure, showing the calorie numbers trail other metrics in reliability olemiss.edu. A 2020 systematic review of 158 publications found reliability varied across nine brands (Apple, Fitbit, Garmin, Mio, Misfit, Polar, Samsung, Withings, Xiaomi) and concluded energy‑expenditure estimates were far less consistent than heart‑rate measurements sciencedirect.com; a 2026 preprint additionally found skin tone and body‑fat percentage materially alter smartwatch calorie estimates, pointing to demographic sources of error arxiv.org. Garmin’s technical notes explain the split between “active” and “resting” calories and that estimates rely on heart‑rate plus accelerometer models garmin.com, while testing guides and lab comparisons show pairing a chest‑strap heart monitor or using a cycling power meter — which records direct mechanical power — measurably reduces calorie‑estimation error for intense or non‑steady activities thetoolstrunk.com.
