Smartwatches can significantly overestimate how many calories people burn during exercise, and their calorie estimates become less accurate as body fat increases, according to a new Florida International University study. The study was published in PLOS One .
FIU researchers tested four popular fitness trackers — the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955 — against clinical-grade laboratory equipment that measured participants' true energy use.
Researchers found typical errors in smartwatch calorie estimates of about 15% to 25%, with averages for the worst-performing watches climbing much higher. Apple was the most accurate. Garmin and Samsung overestimated the most.
That means if a smartwatch says someone burned 500 calories during a workout, the real number could easily be closer to 350. The discrepancy could be especially important for people who use fitness trackers and calorie-burn estimates for weight management.
"You can't treat the calories burned number it gives you as an accurate number, because it's not," said Jason Kostrna , the study's lead author and an associate professor of kinesiology and exercise science in FIU's College of Arts, Sciences & Education . "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."
For the study, researchers had 58 adults ride a recumbent bike through a controlled workout alternating between moderate and vigorous intensity. Each participant wore one of the smartwatches while a COSMED K5 metabolic analyzer, the kind of gold-standard equipment used in clinical labs, measured their true energy use.
The jury is still out on Fitbit. During testing, it produced repeated glitches — in some sessions reporting implausibly small numbers, such as a single calorie for an entire workout, and in others failing to generate a reading at all. Since many of the measurements were erroneous, the researchers filtered out the data.
Kostrna says the team doesn't have a definitive answer for why these trackers struggle with higher body fat percentages. He believes the algorithms used to estimate calorie burn may play a role.
"When the companies are developing those algorithms, we don't know where the test data is coming from," he said. " 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."
Kostrna is now studying how well the watches estimate calories during traditional strength-training workouts.
How to use your smartwatch data
Researchers say smartwatches can still be useful, as long as users understand their limitations. They recommend treating calorie-burn numbers as a ceiling rather than a precise measurement and rounding down for a more conservative estimate. Because the study found accuracy worsened as body fat increased, people using wearables for weight management may want to build in additional margin.
But researchers caution against trying to compensate by cutting food intake too aggressively. The body still needs fuel to recover and train consistently.
Instead of calories burned during a single workout, users can lean more on measurements wearables tend to handle better, including heart rate and time spent in different heart-rate zones. Tracking personal trends over time also can be more useful than focusing on a single day's calorie total.
"Track your own trends over time. And if tracking helps, enjoy it," Kostrna said. "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."
