Smartwatches frequently overestimate calorie burn, study finds

A study conducted by researchers at Florida International University (FIU) has found that popular smartwatches frequently overestimate calorie expenditure during exercise. By comparing four common devices—the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5, and Garmin Forerunner 955—against a clinical COSMED K5 metabolic system, researchers identified typical measurement errors ranging between 15 and 25 percent.

The discrepancies were most notable in the Garmin and Samsung models, which showed the highest upward deviations. While the Apple Watch was found to be closer to the laboratory reference, it still overestimated energy use. The Fitbit Sense 2 produced inconsistent results, including instances where it recorded negligible or zero calories during a workout, leading researchers to exclude much of its data from the final analysis.

Experts note that while the raw sensor data from accelerometers and optical heart rate monitors is generally accurate, the proprietary algorithms used to convert that data into metrics like calories burned, sleep stages, and hydration levels are often imprecise. Researchers suggest that users should view these outputs as tools for tracking long-term trends rather than as precise laboratory-grade measurements.