The higher your body fat percentage, the less accurate your smartwatch is at measuring calories burned
Smartwatches have become an increasingly popular way for people to monitor their health and fitness. About 100 million—nearly 4 in 10—U.S. adults own a smartwatch. Worldwide, market research estimates approximately 560 million smartwatch wearers.
A recent study by researchers at Florida International University's Medical Photonics Laboratory has found that smartwatches often provide inaccurate measurements of calories burned, with the errors increasing as the user's body fat percentage rises. The study, published in the journal PLOS One, examined the accuracy of four popular smartwatch models—Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5, and Fitbit Sense 2—during a standardized cycling session.
Participants included 58 Hispanic adults from Miami, representing a variety of body fat percentages and skin tones. The researchers measured the participants' energy expenditure using a metabolic analyzer, which is considered the most reliable method for determining calorie expenditure. The smartwatches were then compared to this measurement.
The findings revealed that all four devices had substantial errors in their calorie calculations, with the median error ranging from 15% to 25%. However, the errors varied significantly between brands: Apple, Garmin, and Samsung consistently overestimated caloric expenditure, while Fitbit produced implausible estimates, including seven readings exceeding 450% of the actual value.
What was particularly surprising was the relationship between body fat percentage and error. As the participants' body fat percentage increased, the smartwatches became less accurate, with some devices showing overestimates of 50%, 100%, or even more calories burned. This discrepancy is particularly concerning for individuals with higher body fat percentages, who may be relying on these devices to support weight loss efforts.
The study highlights the importance of understanding the limitations of smartwatch technology when it comes to measuring caloric expenditure, especially for those who may be most in need of accurate information for their health and fitness goals.
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