Your smartwatch is lying to you about how many calories you burned in your last workout

If you're depending on your smartwatch to tell you the truth about how many calories you burned during a workout — well, don't.

Fitness trackers are great cheerleaders, delivering tiny doses of motivation with every celebratory chime, buzz and on-screen animation that marks a completed activity goal. But they are less reliable when it comes to measuring total energy expenditure during exercise.

In an independent evaluation, FIU researchers tested four of the most popular fitness watches against gold-standard clinical lab equipment and found they consistently overstated how many calories people burned. And the error wasn't the same for everyone, according to results published in PLOS One.

The higher a person's body fat, the greater the error tended to be — meaning people most likely to track calories for weight management may be getting some of the least accurate numbers.

"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 FIU associate professor of kinesiology and exercise science in the College of Arts, Sciences & Education. "You could be off hundreds of calories each week and easily end up in a calorie surplus when you think you're in a calorie deficit."

For the study, the researchers had 58 adults ride a recumbent bike through a controlled workout that alternated between moderate and vigorous intensity.

Participant wore the most popular brands — Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5, and Garmin Forerunner 955 — while a COSMED K5 metabolic analyzer, the kind of gold-standard equipment used in clinical labs, measured their true energy use.

Typical errors landed in the range of about 15% to 25%, but averages for the worst-performing watches climbed much higher. Apple was the most accurate. Garmin and Samsung overestimated the most.

The jury is still out on Fitbit. During testing, the device occasionally reported implausibly low calorie counts, including just one calorie for an entire workout, and sometimes failed to generate a reading at all. Because many of the measurements appeared faulty, researchers excluded much of the data from their analysis.

Kostrna says the team doesn't have a definitive answer for why these trackers struggle with higher body fat percentages. But 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 already working on the next stage of this research, looking at how well these watches estimate calories during a traditional strength-training workout, where short, intense bursts of effort can be especially tricky for the algorithms to read.

In the meantime, the researchers stress that smartwatches can still be useful, as long as users understand their limitations.

They recommend treating calorie-burn estimates as rough guides rather than precise measurements and rounding down to stay conservative.

At the same time, they caution against trying to make up for the discrepancy by cutting food intake too aggressively. The body still needs fuel to recover and support consistent training.

Users are better off leaning on what wearables track reliably, like heart rate and time in each heart-rate zone, which give you something concrete to act on as you build endurance.

"They may be imperfect, but these devices can still have an important role to play," Kostrna said. "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 feel and use the data as one tool to help you improve over time."