Smartwatches Can Misestimate Calories Burned by 15% to 25%

A 2026 study found several smartwatches misestimated calories burned during cycling, with typical errors around 15% to 25% and larger errors linked to higher body fat.

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Smartwatches can be useful for tracking activity, but their calorie estimates are not as precise as many people assume. A 2026 PLOS One study tested Apple, Garmin, Samsung, and Fitbit watches against indirect calorimetry, a much more direct way to measure energy use, during controlled cycling sessions. The researchers studied 58 adults and found substantial differences between the watches and the reference measurement. Median absolute percentage errors were roughly 15% to 25%, while Garmin and Samsung showed a consistent tendency to overestimate energy expenditure. Fitbit produced the widest variation, including several extreme readings, while Apple showed a smaller average bias in this particular test. The study also found that calorie-estimation error increased as body fat percentage increased across the devices. Skin tone did not show a robust independent effect in this sample, although the researchers caution that the sample was limited and the finding should not be generalized to every real-world situation. The experiment involved one controlled cycling protocol, so accuracy could differ during walking, running, strength training, housework, or everyday movement. The broader lesson is that a watch calorie number is an estimate rather than a direct measurement. Earlier reviews of wearable devices have also found that energy-expenditure accuracy varies substantially by device and activity, reinforcing the idea that these numbers are better treated cautiously than as exact measurements.

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