Smartwatches overestimate calorie burn by up to 25 percent

A study published in the journal PLOS One by Florida International University reveals that popular smartwatches systematically overestimate calorie expenditure. Researchers compared four devices—the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5, and Garmin Forerunner 955—against a COSMED K5 metabolic analyzer during exercise on a recumbent bicycle.

The investigation found that calorie estimates were generally 15 to 25 percent higher than actual values, with some discrepancies being even more significant. Accuracy tended to decrease as the body fat percentage of participants increased; for instance, a displayed value of 500 kilocalories often corresponded to a real expenditure of only approximately 350 kilocalories.

Among the tested devices, the Apple Watch Series 8 performed most accurately. In contrast, the Fitbit Sense 2 provided unusable data, including errors such as recording only 1 kilocalorie per session. Dr. Jason Kostrna, the study's author, advised users to treat smartwatch calorie readings as an upper limit and suggested focusing on heart rate or heart rate zones for more reliable training guidance.