Smartwatches May Overestimate Calories Burned During Exercise

Why Body Fat Affects Calorie Readings

Researchers at Florida International University found that popular fitness trackers often report higher calorie burn than actual energy expenditure, with errors increasing for individuals with higher body fat. The study, published in PLOS One on September 1, 202026, evaluated four widely used smartwatch models during controlled exercise sessions. Participants performed treadmill walking and running while wearing the devices, with their true calorie burn measured via indirect calorimetry, the gold standard for metabolic assessment. Results showed consistent overestimation across all devices, particularly during moderate-intensity activity, and the discrepancy grew as body fat percentage rose. Researchers suggest that algorithms used by these wearables may not adequately account for variations in body composition when estimating energy use. The findings raise concerns about reliance on smartwatch data for weight management or fitness tracking, especially among populations with higher adiposity.

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The study indicates that smartwatch algorithms often rely on heart rate and motion sensors to estimate calories burned, assuming a standard relationship between physiological signals and metabolic rate. However, individuals with higher body fat may exhibit different cardiovascular responses to exercise, leading to skewed interpretations by the devices. As adipose tissue does not consume energy at the same rate as muscle during activity, the models may misattribute effort or overcompensate in their calculations. This limitation becomes more pronounced when users engage in prolonged or varied-intensity workouts, where individual physiological differences play a larger role. The researchers emphasize that while wearables offer convenience, their estimates should be interpreted cautiously, particularly for health or clinical applications requiring precise energy expenditure data.

Can Users Trust Their Fitness Trackers for Weight Loss Goals?

Given the tendency to overestimate calorie burn, users might unknowingly consume more calories than they have actually expended, potentially hindering weight loss efforts. The study warns that relying solely on device feedback for dietary or exercise adjustments could lead to ineffective or counterproductive outcomes. Experts recommend combining wearable data with other metrics, such as perceived exertion or heart rate zones, and consulting professionals when setting fitness targets. Future improvements in sensor technology and personalized algorithm calibration may enhance accuracy, but current models remain limited by population-based assumptions. Until then, users should treat smartwatch calorie readings as rough guides rather than precise measurements.

How much do smartwatches typically overestimate calorie burn? In the study, devices overestimated calorie expenditure by an average of 15% to 25% during moderate-intensity exercise, with higher errors observed in participants with elevated body fat levels.

Frequently Asked Questions

Does this affect all types of workouts equally? Overestimation was most consistent during steady-state aerobic activities like walking and jogging; results varied more during high-intensity interval training, though accuracy remained suboptimal across all tested modalities.

Are any smartwatch brands more accurate than others? All four devices tested showed similar patterns of overestimation, with no significant difference in accuracy between brands, suggesting the issue stems from shared methodological limitations in current wearable technology.