Published: 13:01 BST, 2 September 2026 | Updated: 13:05 BST, 2 September 2026
Apple, Samsung and Garmin smartwatches overestimate the number of calories you burn by at least 110, US scientists have discovered.
Fitbit watches, meanwhile, are 'consistently glitchy' and produce both dramatic over and underestimations.
Garmin watches are most likely to overshoot energy expenditure, while the Apple device is the most accurate.
Those with a high level of body fat are more likely to get inaccurate results, according to the researchers, from Florida International University.
Smartwatches are among the most popular types of wearable technology, with a third of Britons choosing to invest in one, according to YouGov polls.
Around one in five Britons have the type that tracks fitness measurements, such as calorie counts, heart rate and sleep quality.
But experts have warned users not to rely on the devices to monitor their weight loss plan.
'You can't treat the calories burned number it gives you as accurate, because it's not,' said Jason Kostrna, the study's lead author and associate professor of kinesiology and exercise science at Florida International University.
'You can't treat the number of calories burned as accurate, because it's not, researchers say
'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. '
Prof Kostrna added that 'improvements to the device or algorithm are needed'.
In his study, published in the journal PLOS One, the researchers asked 58 adults to ride a special stationary bike whereby cyclists sit in a reclined position.
Each participant did 60-minutes of cycling – a mixture of both moderate and vigorous exercise – while wearing one of four smartwatches; the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955.
Volunteers were also fitted with a so-called metabolic analyser, which tracks calories burned by measuring how much oxygen is used while exercising.
