A study published in PLOS One has found that popular smartwatches can substantially overestimate calorie expenditure during exercise, with typical errors ranging from 15% to 25%, and accuracy declining as body fat percentage increases. The research, conducted at Florida International University, has particular implications for individuals using wearable fitness trackers for weight management.
Researchers tested four commercially available devices for measuring energy expenditure. Fifty-eight adults completed a controlled recumbent cycling protocol alternating between moderate and vigorous intensity while wearing one of the devices alongside the metabolic analyser.
The authors note that if a device reports 500 calories burned during a workout, the actual figure could be closer to 350, a discrepancy that could result in a user inadvertently maintaining a calorie surplus while believing they are in a deficit. Accuracy was found to worsen as body fat percentage increased, which the researchers attribute in part to the proprietary algorithms used by manufacturers, the training data for which is not publicly disclosed.
The researchers advise treating calorie estimates as an upper ceiling rather than a precise figure, and suggest that current wearable devices may more reliably measure heart rate data and time spent in heart rate zones.
Source: Kostrna J et al. Body fat, skin tone, and the accuracy of smartwatch caloric expenditure estimates. PLOS One (2026). DOI: 10.1371/journal.pone.0353261
