Smartwatch users should be cautious about relying too heavily on the calorie-burning numbers these devices provide. With nearly 4 in 10 U.S. adults owning a smartwatch and an estimated 560 million users worldwide, these devices have become a popular tool for tracking health and fitness. However, recent research suggests that one of the most frequently used metrics—calories burned—may be significantly inaccurate. A study conducted by a team of researchers at Florida International University's Medical Photonics Laboratory examined how well four popular smartwatches—Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5, and Fitbit Sense 2—measure energy expenditure during exercise. The researchers tested 58 Hispanic adults from Miami, who had a wide range of body fat percentages and skin tones. Participants performed a standardized cycling session while wearing the devices, and their actual caloric expenditure was measured using a metabolic analyzer, which tracks the gases inhaled and exhaled through a mask. This method is considered the most accurate way to measure calorie burn. The study found that all four smartwatches had substantial errors in measuring caloric expenditure, with median errors ranging from 15% to 25%. Apple, Garmin, and Samsung all overestimated the calories burned, while Fitbit had instances where the readings were extremely inaccurate, with some estimates exceeding 450% of the actual value. The researchers were surprised by how strongly the accuracy of the devices was linked to the user's body fat percentage. As body fat increased, the errors in the devices’ calorie measurements also increased, though the degree of inaccuracy varied by brand. While the general inaccuracy of smartwatches in measuring calories is not new, the study highlights that people with higher body fat percentages—who are often the ones most interested in using these devices for weight management—may receive the least accurate data. Small errors in individual workouts can add up over time, potentially leading to significant miscalculations in daily caloric expenditure. For someone with a high body fat percentage who exercises regularly, this could mean underestimating or overestimating their caloric needs by hundreds of calories per week. Despite these limitations, smartwatches still offer value in tracking other metrics such as heart rate, workout duration, and pace. Researchers advise users to treat the calorie numbers as estimates rather than exact figures, especially when managing weight. Consistency in the numbers does not guarantee accuracy, and users are encouraged to consider other factors like heart rate, exercise intensity, and personal experience when assessing their fitness progress. As wearable technology continues to evolve, improving the accuracy of caloric expenditure across different body types may become a key focus for future development.