Smartwatches Significantly Overestimate How Many Calories Are Burned

TUESDAY, Sept. 8, 2026 (HealthDay News) — Smartwatches significantly overestimate how many calories people burn during exercise, according to a study published online July 29 in PLOS One.

Jason Kostrna, from Florida International University in Miami, and colleagues evaluated the accuracy of smartwatch-derived physical activity energy expenditure estimates compared to indirect calorimetry, and examined whether error varied by device brand, body fat percentage, and skin tone. The analysis included participant-device pairings: 52 participants wearing Apple Watch Series 8; 51 wearing Garmin Forerunner 955; 50 wearing Samsung Galaxy Watch 5; and 44 wearing Fitbit Sense 2.

The researchers found significant mean bias for three of four devices, with the nonsignificant Fitbit bias depending on device-specific outlier removal. Errors in smartwatch calorie estimates ranged roughly from 15 to 25 percent, with averages for the worst-performing watches climbing higher (Apple mean: 21.60 kcal [95 percent confidence interval (CI), 11.59 to 31.60]; Garmin: 68.61 kcal [95 percent CI, 53.28 to 83.94]; Samsung: 56.76 kcal [95 percent CI, 45.11 to 68.41]; Fitbit: 3.14 kcal [95 percent CI, −8.96 to 15.24]). There were main effects for device, body fat percentage main, and a device-by-body fat percentage interaction, according to mixed-effects models. Across all brands, physical activity energy expenditure error increased with adiposity.

“You can”t treat the calories burned number it gives you as an accurate number,” Kostrna said in a statement. “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.”

Abstract/Full Text