We all know smartwatch and fitness-tracker calorie estimates aren’t perfectly accurate. They give a useful approximation for activity and trends, but recent research suggests the inaccuracy may be larger than many of us expect—especially for people with higher body fat percentages.
A new study compared calorie estimates from several popular smartwatches to a medical-grade device that measures actual energy expenditure. The researchers found systematic errors in many devices’ calorie estimates, and those errors worsened as body fat percentage increased. For anyone using a tracker to manage weight, exercise, or daily energy balance, that finding is important.
How the Study Worked
The research recruited adult participants who completed structured exercise tests on a stationary bike, alternating between moderate and vigorous intensity. During these sessions, participants wore multiple consumer smartwatches simultaneously while a research-grade metabolic measurement system recorded true energy expenditure. The study aimed to compare the watches’ reported calorie burn with objective measurements under controlled conditions.
The brands included in the test were among the most widely used, and three of the four devices in the study tended to overestimate calories burned. Average errors typically ranged from about 15 to 25 percent, while one brand showed more variable results. Crucially, the degree of error tracked closely with participants’ body fat percentage: the higher the body fat, the bigger the discrepancy. In some cases among participants with higher body fat, calorie estimates exceeded measured calories by 50 to 100 percent or more.
Real-World Impact
To put these discrepancies into perspective, the researchers translated percentage errors into weekly calorie totals. For example, someone with a higher body fat percentage who exercises several hours per week could see cumulative overestimates of a thousand or more calories across seven days. Over time, using inflated calorie-burn figures to justify increased eating or reduced calorie restriction could undermine weight-loss goals or distort daily energy accounting.
The study’s lead researcher noted that companies typically do not disclose the exact algorithms used to calculate calorie expenditure, which makes it difficult to pinpoint the technical causes of the bias. Factors such as sensor placement, heart-rate algorithms, individual physiology, and how devices estimate metabolic equivalents may all play a role, but the study focused on documenting the discrepancy rather than reverse-engineering commercial algorithms.
What You Can Do
- Treat calorie-burn estimates as rough guides rather than precise measurements. Use them for trends and relative comparisons (for example, seeing whether one workout felt more intense than another) rather than exact calorie math.
- If you rely on a tracker for weight management, consider using the calorie number conservatively—round down or use a buffer when adjusting food intake based on exercise burn.
- Track other reliable metrics the devices measure well, such as heart rate, workout duration, distance, and pace. These data points can help you monitor fitness progress without depending solely on calorie estimates.
- Combine tracker data with other methods: regular weigh-ins, body measurements, and how your clothes fit provide practical feedback that balances sensor estimates.
- Consult a registered dietitian or certified fitness professional for personalized guidance if you depend on precise energy calculations for clinical or competitive reasons.
What This Means for Manufacturers and Users
The study highlights a clear need for transparency and continued improvement in how wearables estimate energy expenditure. For manufacturers, validating algorithms across diverse body types and publishing more details about performance could improve consumer trust and product utility. For users, the takeaway is straightforward: smartwatches remain valuable tools for monitoring activity and motivating movement, but their calorie estimates should be interpreted cautiously—especially for people with higher body fat percentages.
In short, smartwatches can accurately track many aspects of fitness, but when it comes to calories burned during exercise, consider the numbers directional. Use them to guide behavior and track trends over time, but avoid treating a single session’s calorie readout as a definitive measurement of energy expended.