Three of four popular smartwatches significantly overestimated how many calories their wearers burned during exercise, and the size of the error grew as body fat percentage rose, according to a Florida International University study publicised on 1 September 2026. The paper was published in PLOS ONE on 29 July 2026. Researchers tested the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955 against a COSMED K5 metabolic system, the laboratory standard for measuring energy expenditure. Fifty-eight adults completed a 10-minute recumbent cycling protocol alternating moderate and vigorous intervals. Garmin was furthest out, overestimating by an average of 68.61 calories across that short session, followed by Samsung at 56.76 and Apple at 21.60. Lead author Jason Kostrna said wearers "could be off hundreds of calories each week and easily end up in a calorie surplus when you think you're in a calorie deficit".
The study is small, young and narrow
The limitations matter. The 58 participants had a mean age of 23, all identified as Hispanic, and all had Fitzpatrick skin types III to V, with only four at type V. The researchers tested a single exercise mode in a single session, so they could not see whether the devices' algorithms adapt over time, and they note that wearing four watches at once may have affected sensor contact. Funding came from a National Science Foundation pilot grant and the authors declared no competing interests. Fitbit's average bias was the smallest at 3.14 calories, but the FIU release says it sometimes returned implausible readings, including one calorie for a whole workout, or none at all.
What this means in practice
The same four devices are sold in the UK, so the finding travels, with the caveat that this was a young, ethnically narrow sample doing one type of cycling. The practical point the researchers make is that the calorie figure is the least reliable number on the watch face, while heart rate and heart rate zones held up better. The error was largest in participants with higher body fat, which is the group most likely to be using the calorie readout in the first place. Anyone budgeting food intake against a watch's estimate is working from a number the authors describe as a rough guide rather than a measurement.
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