Smartwatches regularly overestimate workout calorie burn by 15% to 25%, with errors worsening as body fat percentages rise, according to a study published in PLOS One. Researchers testing four popular fitness trackers against clinical laboratory equipment found that devices like Garmin and Samsung often inflated energy expenditure figures significantly during exercise.
Checking a wearable device after a hard gym session to calculate post-workout meals feels like reasonable math. But recent research from Florida International University reveals that the calorie counts displayed on popular smartwatches are frequently untreliable. For people relying on fitness trackers to manage weight, these inaccurate metrics can quietly sabotage dietary goals.
How Researchers Tested Popular Fitness Trackers Against Lab Equipment
To evaluate how well wearable technology measures energy use, researchers recruited 58 adult volunteers. Participants completed a controlled recumbent bicycle workout that alternated between moderate and vigorous intensities. While cycling, each participant wore one of the most popular consumer fitness devices on the market: the Apple Watch Series 8, Samsung Galaxy Watch 5, Fitbit Sense 2, or Garmin Forerunner 955.
To establish a baseline of true physiological effort, the team used a COSMED K5 metabolic analyzer. This clinical-grade laboratory equipment—the kind of gold-standard equipment used in clinical labs—measured actual energy use. Comparing the watch readouts against the metabolic analyzer data exposed stark discrepancies across nearly every brand.
Apple, Garmin, and Samsung Performance Compared
The study found that typical errors in smartwatch calorie estimates landed between 15% and 25%, with worst-performing devices exceeding those averages substantially. Among the tested hardware, Apple Watch Series 8 delivered the smallest average systematic error, proving to be the most comparatively accurate option. Meanwhile, Garmin Forerunner 955 and Samsung Galaxy Watch 5 produced the most generous overestimations.
Fitbit Sense 2 presented an entirely different technical hurdle during the trial. The device produced repeated glitches, occasionally logging implausibly small numbers—such as a single calorie burned for an entire workout—or failing to generate any reading at all. Because these erratic measurements compromised data integrity, researchers had to filter out much of the Fitbit data from their primary analysis.
The Hidden Complications of Body Composition and Proprietary Algorithms
The discrepancy between actual energy expenditure and watch readouts grows even wider depending on who is wearing the device. Data showed that error magnitudes increased as body fat percentages rose. This creates a difficult irony: the individuals most motivated to track caloric burn for weight management are receiving the least reliable estimates.
Wearables successfully measure accessible signals like movement via accelerometers and heart rate through light-based wrist sensors. But calculating total energy expenditure requires software to predict complex physiological realities that hardware cannot observe directly. Because manufacturers guard their software architecture closely, independent researchers have little visibility into how those calculations are built.
Kostrna noted that sharing such laboratory data publicly could help manufacturers refine their products over time.
What This Means for Weight Management and Dieting Goals
Relying on inflated workout metrics carries genuine nutritional consequences. If a wearable claims an individual burned 500 calories during a session, the true figure could easily sit closer to 350.
“You could be hundreds of calories off each week and easily end up in a calorie surplus when you think you’re in a calorie deficit.”
Jason Kostrna, lead author and associate professor, Florida International University
Shifting Focus From Single-Workout Calories to Long-Term Trends
As research continues, the team behind the study is already preparing the next stage of investigation, which will analyze how accurately these same consumer smartwatches estimate energy expenditure during traditional strength-training workouts characterized by short, intense bursts of effort.