Smartwatches Vastly Overestimate Calorie Burn During Workouts, Study Finds

New research shows that popular wearables could be misleading consumers — especially those with higher body fat percentages — about how many calories they’re actually burning during workouts, a potential issue for the “track-everything” movement

While most people probably look at their smartwatch metrics knowing they’re not the most precise measurements, consumers love to dial into data like their heart rate variability (HRV), recovery, sleep and, arguably the most eye-catching number, calories.

But new research suggests that the all-important calorie-burn estimate your watch provides could be significantly overestimated.

A study out of Florida International University reveals that calorie burn estimates become less accurate as body fat increases, with the margin of error anywhere between 15-25% depending on the brand and model.

To test the devices, researchers put 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’ accurate energy use. 

They found Apple to be the most accurate to the lab equipment’s measurements, with Garmin and Samsung having the most egregious overestimates. That margin of error only worsened for participants with higher body fat percentage, which could make it more difficult for those who depend on their devices for weight management and calorie tracking.

“You can’t treat the calories burned number it gives you as an accurate number, because it’s not,” said lead study author and an associate professor of kinesiology and exercise science at FIU, Jason Kostrna.  “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.”

The 58 participants went through a guided interval workout on a recumbent bike alternating between moderate and vigorous intensity, with each participant wearing one of the smartwatches while clinical-grade lab equipment also measured calorie expenditure.

Fitbit’s measurements proved inconclusive, as the device produced repeated glitches or no calorie readings at all in some sessions.

As weight loss only becomes all the more societally relevant with the ubiquity of GLP-1s, these findings could reshape how people approach their health journeys. Calorie intake is especially important for GLP-1 users, whose suppressed appetites mean their diets have to be all the more dialed in to make sure they meet their nutritional and caloric needs.

Kostma was uncertain as to why the watches struggled to estimate calorie burn, pointing to the algorithms as a potential culprit.

“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.”