The higher your body fat percentage, the less accurate your smartwatch is at measuring calories you burn when exercising – new study

Researchers at Florida International University tested four popular smartwatches, the Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5 and Fitbit Sense 2, against a metabolic analyzer during controlled cycling sessions with 58 adults.

All four devices showed substantial error, with median errors of 15 to 25 percent, and Apple, Garmin and Samsung all systematically overestimated calories burned. Fitbit's errors weren't consistently biased but included several wildly implausible readings, including seven exceeding 450 percent of the true value.

The key finding was that error grew alongside a wearer's body fat percentage, ranging up to 50 to 100 percent or more for those with higher body fat, a group especially likely to be using the watch to track weight loss. Skin tone showed no significant effect under the study's conditions.

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