Study Finds Popular Fitness Smartwatches Overestimate Calorie Burn by Up to 25%

In a controlled laboratory study, Florida International University evaluated four widely used fitness smartwatches by having 58 adults cycle on a recumbent bike while wearing each device. The watches' calorie-burn readings were benchmarked against a COSMED K5 metabolic analyzer, considered the gold standard for measuring true energy use. Results indicated that every smartwatch inflated the calories burned, with typical discrepancies of 15% to 25%; Apple Watch Series 8 performed best, whereas Garmin Forerunner 955 and Samsung Galaxy Watch 5 showed the greatest overestimation.

Fitbit Sense 2 occasionally logged absurdly low numbers, even as low as one calorie for an entire session, and sometimes failed to produce a reading. Because of these inaccuracies, the study’s lead author, Jason Kostrna, recommends users view smartwatch calorie data as a general guide rather than an exact figure. The findings highlight the need for caution when relying on wearable devices for precise dietary or training decisions.