The calorie count your smartwatch shows after a workout may be off by 15 to 25 percent, according to new research from Florida International University. Across four leading models, every device tested misjudged how much energy the wearer actually burned during exercise.
The team compared the Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch 5 and Fitbit Sense 2 with a laboratory-grade metabolic analyzer. The Apple Watch came closest to the reference measurement. Garmin and Samsung overestimated the most. Fitbit's readouts were largely dropped from the analysis because they sometimes recorded a single calorie for an entire session.
The findings appear in PLOS ONE, which published the peer-reviewed paper in July 2026.
Where the numbers come from
The researchers recruited 58 adults for a recumbent cycling session that alternated between two-minute bouts at moderate and vigorous intensity, bookended by short rest and recovery periods. A COSMED K5 metabolic analyzer, which measures oxygen consumed and carbon dioxide produced, provided the criterion reading for calories burned.
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When the wrist readouts were compared with the lab measurement, Garmin averaged about 69 calories too high, Samsung roughly 57 too high and Apple about 22 too high. The gap between device and lab widened as the participant's body fat percentage rose, a pattern seen across every brand. Skin tone, measured on the Fitzpatrick scale, did not show a significant effect within the range studied.
The Florida International team said the error grew with adiposity across all four brands, meaning the people most likely to lean on the readout for weight management may be seeing the least reliable numbers.
Why the wristwear falls short
Consumer trackers estimate calories from a mix of heart-rate data, motion sensors and personal inputs like age, height and weight, then run those inputs through a company-proprietary algorithm. A 2017 Stanford University study led by cardiologist Euan Ashley found the same divide with an earlier generation of wristbands. Heart rate came within about 5 percent of a chest-strap reference, but every device tested missed energy expenditure by at least 27 percent, and one by 93 percent.
"When the companies are developing those algorithms, we don't know where the test data is coming from," Jason Kostrna, associate professor of kinesiology and exercise science at Florida International University and the study's lead author, told FIU News.
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What to do with the number on your wrist
Kostrna's practical advice is to treat the calorie readout as a rough guide rather than a ledger entry, and to avoid aggressive cuts in food based on what the watch says.
"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," Kostrna said in a Florida International University release.
The same devices track heart rate reliably. Watching heart rate and time in specific heart-rate zones during a workout is a firmer anchor than the calorie estimate. The Florida International study focused on stationary cycling, and earlier work has found similar gaps for running and walking, so switching activity is unlikely to close the difference.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.