That calorie number on your smartwatch may look precise, but new research suggests it is better treated as an estimate.
In a study of 58 adults, researchers compared calorie-burn estimates from four popular smartwatches with laboratory measurements taken during a controlled cycling workout. All four devices showed meaningful errors, although some performed better than others. The watches also tended to become less accurate as participants’ body fat percentage increased.
The study, published in PLOS One, tested the Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch 5 and Fitbit Sense 2. Participants wore the watches while cycling at moderate and vigorous intensities, while researchers used laboratory equipment to measure how much energy they were actually using.
The gap between the watches and the laboratory measurements was often substantial. Median percentage errors generally ranged from about 15% to 25%.
Apple came closest overall, while Garmin and Samsung tended to overestimate calories burned. Fitbit was harder to judge because it produced several extremely inaccurate readings and occasionally failed to provide an estimate at all. When those implausible readings were removed, its average looked much better, but the researchers cautioned that the device still showed considerable variability.
That means the number on your wrist may be useful as a rough guide, but not necessarily as an exact accounting of how many calories a workout burned.
Jason Kostrna, the study’s lead author and an associate professor at Florida International University, put it simply: “You can't treat the calories burned number it gives you as an accurate number, because it's not.”
The study also found that calorie estimates became less accurate as body fat percentage increased. The pattern showed up across all four devices, although the size of the error varied by brand. Researchers could not say exactly why, in part because the formulas companies use to calculate calorie burn are proprietary.
The researchers also looked at skin tone and did not find a clear effect on calorie estimates. But that finding should be interpreted cautiously. All participants were Hispanic adults with a limited range of skin tones, and only four people were in the darkest group included in the study. That means the results cannot rule out differences in other populations or in everyday use.
The study has other limitations. Participants completed only one exercise session, all on a recumbent bicycle, so the exact error rates may not apply to walking, running, strength training or normal daily activity. The watches were also worn under controlled laboratory conditions, which may differ from how people use them in real life.
Still, the results add to a growing body of research suggesting that fitness trackers are better at some measurements than others. A calorie total may look exact because it appears as a single number, but it is still calculated from sensor data and an algorithm.
That matters most for people using wearable calorie estimates to guide how much they eat or whether they believe they are in a calorie deficit. If the watch overestimates what a workout burned, someone could easily think they have more room to eat than they actually do.
The answer, though, is not to stop using a smartwatch or to automatically subtract a certain percentage from every calorie total. This study does not establish a universal correction factor, and not every watch overestimated every workout.
The researchers instead suggest treating calorie-burn numbers as one piece of information rather than a precise measurement. Looking at patterns over time may be more useful than focusing on whether a single workout supposedly burned 437 calories instead of 389.
Kostrna recommended that same broader view, saying, “Track your own trends over time. And if tracking helps, enjoy it.”
For people who like fitness data, that may be the most useful takeaway: the watch can still help show how active you are, but its calorie number deserves a little skepticism.
The research was supported by a pilot grant from the National Science Foundation through the Precise Advanced Technologies and Health Systems for Underserved Populations, or PATHS-UP, program.