Your Smartwatch Is Lying About Calories — And the People Who Need It Most Are Getting the Worst Data

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If you’ve ever finished a workout, checked your watch, and mentally banked those calories toward dinner, you might want to sit down. A new Florida International University study published in PLOS One found that smartwatches significantly overestimate how many calories you burn during exercise — and the problem gets worse depending on who’s wearing the device.

The Study: What They Tested

Researchers had 58 adults ride a recumbent bike through a controlled workout alternating between moderate and vigorous intensity. Each participant wore one of four popular smartwatches while a COSMED K5 metabolic analyzer — the kind of clinical-grade equipment used in labs — measured their actual energy expenditure .

The devices tested:

Apple Watch Series 8
Samsung Galaxy Watch 5
Garmin Forerunner 955
Fitbit Sense 2

The Results: 15% to 25% Wrong — Or Worse

Every watch showed substantial errors, typically off by 15% to 25% . But the averages tell a more specific story:

Device	Average Overestimation
Garmin Forerunner 955	~68.6 calories
Samsung Galaxy Watch 5	~56.8 calories
Apple Watch Series 8	~21.6 calories
Fitbit Sense 2	Unreliable — 13% of readings were implausible, including some that logged a single calorie for an entire workout

Put simply: if your watch says you burned 500 calories, the real number could easily be closer to 350 .

“You can’t treat the calories burned number it gives you as an accurate number, because it’s not. 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 of kinesiology at FIU

The Cruel Twist: The People Who Need It Most Get the Worst Data

Here’s where it gets personal. The study found that error magnitude increased as body fat percentage rose across all four brands . That means higher-body-fat users — often the people most motivated to track calories for weight management — are working with the shakiest estimates the watch produces .

The reasons aren’t fully understood. Kostrna suspects the proprietary algorithms used to estimate calorie burn may play a role, but the companies won’t disclose their training data or methodology .

“When the companies are developing those algorithms, we don’t know where the test data is coming from. That’s why I think it’s beneficial for the companies to see this type of data, so they can refine their products over time.”
— Jason Kostrna

The study also examined skin tone as a potential factor. The good news: researchers found no noticeable variation in accuracy across different skin tones . The bad news: body composition matters far more than the industry has acknowledged.

The Bigger Picture: Smartwatch Metrics Aren’t What They Seem

The calorie issue is part of a broader problem. A University of Michigan study published this month found that smartwatch metrics exist on a spectrum — some are relatively reliable, others are algorithmic guesses dressed up as precise data .

More reliable: Heart rate, step count, and outdoor pace — these are closer to what the sensors actually detect.

Less reliable: Calories burned, sleep stages, body composition, hydration, and recovery scores — these are estimates generated by combining sensor signals with proprietary algorithms and assumptions .

“People shouldn’t take these metrics at face value. In many cases, these devices are better suited to tracking trends over time, rather than as precise laboratory measurements.”
— Adam Lepley, assistant professor at the University of Michigan School of Kinesiology

Accuracy can also be affected by movement, watch fit, temperature, sweat, tattoos, and body composition. Results may not be comparable across brands because manufacturers use different sensors, definitions, and algorithms .

What You Can Do

1. Stop budgeting meals around your calorie burn. If your watch says you burned 500 calories, round down — way down. Treat the number as a ceiling, not a precise measurement .

2. Focus on trends, not single readings. A consistent shift in resting heart rate or sleep over time is more meaningful than any one workout’s calorie total .

3. Lean on heart rate zones. Researchers say heart rate and time spent in different heart-rate zones are more reliable metrics for guiding training than calorie counts .

4. Don’t compensate by cutting food too aggressively. The body still needs fuel for recovery and consistent training. Undereating based on overestimated burn numbers can backfire .

“Track your own trends over time. If tracking helps, enjoy it. If it becomes too much, don’t fixate on it. Focus on how you’re feeling and use data to get a little better over time.”
— Jason Kostrna

The Bottom Line

Your smartwatch is a useful compass, not a laboratory instrument. It can help you build habits, track progress, and stay motivated. But the calorie number — especially if you’re carrying more body fat — is closer to a guess than a measurement.

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