Leading Smartwatch Brands Overestimate Calories Burned by 25 Percent

New research shows popular smartwatches can overestimate calories burned during exercise by 15–25 percent, with Apple Watch the most accurate and Garmin and Samsung the least. In controlled tests against clinical-grade metabolic analyzers, some devices were so unreliable (Fitbit Sense 2) that researchers had to discard data because of glitches.

The miscalculations matter for weight management: a claimed 500-calorie workout is closer to 350, potentially turning an intended calorie deficit into a surplus over time. Accuracy also tended to worsen for wearers with higher body-fat percentages, likely because companies’ proprietary algorithms are built on opaque training data. Researchers recommend treating smartwatch calorie numbers as optimistic estimates and focusing on overall trends and how you feel rather than precise figures. 

“You can’t treat the calories burned number it gives you as an accurate number, because it’s not,” said Florida International University kinesiologist Jason Kostrna.

Kostrna and his colleagues compared four of the most popular fitness wearables on the market: the Apple Watch Series 8, Samsung Galaxy Watch 5, Fitbit Sense 2, and Garmin Forerunner 955 against clinical-grade measurement equipment. They then asked 58 adult volunteers to use a stationary bike during a controlled workout session, while alternating between moderate and intense activity. They recorded the results simultaneously with one of the commercially available wearables and a COSMED K5 metabolic analyzer. For reference, COSMED fitness equipment appears during James Bond’s physiology assessment scene in the 2012 film Skyfall.

According to results recently published in the journal PLOS One, smartwatch calorie overestimations ranged between 15 and 25 percent. Apple was the most comparatively accurate, while Garmin and Samsung offered the most generous results. Meanwhile, the researchers needed to discard Fitbit from their trial entirely because the device repeatedly glitched during experiments. In some instances, the Fitbit recorded a single calorie burned during an entire workout, or even failed to generate any assessment.

To put the larger findings in perspective: if your wearable claims you burned 500 calories at the gym, that number may be much closer to 350 calories. This may not seem too serious, but the errors can easily add up over time.

“You could be hundreds of calories off each week and easily end up in a calorie surplus when you think you’re in a calorie deficit,” said Kostrna.

Tests also consistently indicated that smartwatches become less accurate for wearers with higher body fat percentages. The exact reason is still unclear, but Kostrna’s team suspects the issue may stem from how each device’s algorithms are calculated. Tech companies often keep this information closely guarded from public scrutiny.

However, this doesn’t mean the devices are completely useless. Their calorie measurements should be treated as generous estimates. Researchers suggest treating a reading as a best-case scenario after each workout, while people with a higher body fat index may want to view the numbers even more conservatively. At the same time, the study’s authors warn against solutions like more aggressive dieting, since the body still requires healthy foods to recharge and improve. In any case, Kostrna stressed that calorie monitoring should only be one part of a person’s larger fitness goals.