Research from Florida International University shows that popular wearable devices often overstate calories burned during exercise, with errors increasing alongside body fat percentage, raising concerns for users managing weight or tracking fitness.
Smartwatches are now common enough to sit on the wrists of tens of millions of Americans, but a new Florida International University study suggests one of their most popular readings should be treated with caution. The research found that calorie-burn estimates from four widely used devices can be materially wrong, and that the error becomes larger as body fat percentage rises. According to reporting on the study, the work was published in PLOS One on 29 July 2026 and examined Apple, Garmin, Samsung and Fitbit devices.
The FIU team tested 58 Hispanic adults in Miami, choosing a group with a broad range of body fat levels and skin tones. Participants completed controlled cycling sessions on a recumbent bike while wearing an Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5 and Fitbit Sense 2. Their energy expenditure was measured with indirect calorimetry, the laboratory method widely regarded as the reference standard in exercise science.
The results were not encouraging. Three of the four watches showed a clear tendency to overstate calories burned, with typical errors in the range of about 15% to 25%. Fitbit was less consistently biased in the average, but produced several implausible readings that the researchers treated as device errors. The study found no meaningful effect from skin tone under the conditions tested.
The most important finding was that body fat percentage was closely linked to inaccuracy. Across all four brands, error increased as adiposity rose, and in some cases the discrepancy reached roughly 50% to 100% or more for participants with higher body fat levels. The researchers said they could not determine precisely why this happened, although the design of proprietary algorithms and the way devices are calibrated are likely to matter.
For consumers, the practical lesson is straightforward. Heart-rate and movement data from smartwatches can still be useful, and the same FIU team has found those measures to be reasonably accurate in a companion study. But the calorie figure should be treated as an estimate, not a precise measure of energy expenditure. That warning is especially relevant for people using wearables to manage weight, where even modest daily errors can add up over time.
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