What the research shows
Fitness trackers are good at some things and poor at others. Studies agree on the pattern:
- Heart rate is mostly accurate. In a Stanford study of seven wrist devices worn by 60 people, six measured heart rate with a median error under 5% during cycling.
- Calories are not. In the same study, no device estimated calories burned to within 20%. The Apple Watch had the lowest error and the Samsung Gear S2 the highest, but even the best was well outside the 10% the researchers considered reasonable for everyday users.
- No brand gets it consistently right. A 2020 review of 43 studies of calorie estimates concluded that for energy expenditure, no brand was accurate. Depending on the brand and activity, devices both overestimated and underestimated.
- Activity type matters. A 2020 meta-analysis of 60 studies found errors differed by activity, with devices tending to underestimate cycling. Devices that combine heart rate with motion sensing did better than motion alone.
- Your body matters too. A 2026 study of current watches from Apple, Fitbit, Samsung and Garmin found significant errors for three of the four, and errors grew larger as body fat increased.
Active calories vs total calories
Much of the confusion comes from two different numbers. Total calories include the energy your body would use anyway, just to keep running. Active calories leave that out and count only the extra energy from moving.
A 30 minute brisk walk might be about 175 calories in total but only about 140 active calories for a 155 lb person. Comparing an active number from one device with a total number from another, or from a calculator, makes them look far apart even when both are reasonable. The tool above lets you choose which kind your tracker shows.
Brand by brand
Most studies test older models, and results swing between activities, so no brand has a clear lead. With that caveat, the 2020 review of 43 studies found these tendencies in lab settings:
- Apple Watch overestimated in 58% of comparisons. See how accurate Apple Watch calories are.
- Fitbit was the only brand whose average fell within acceptable limits, but individual results varied widely in both directions. See how accurate Fitbit calories are.
- Garmin underestimated in 69% of comparisons.
- Polar overestimated in 69% of a smaller number of comparisons.
Newer devices such as smart rings and strap-style trackers have far fewer independent studies of calorie accuracy. Until there are more, it is reasonable to assume they share the same uncertainty.
How to get a more useful number
- Keep your height, weight, age and sex up to date in the app. Every estimate starts from them.
- Wear the watch snugly, a finger's width above the wrist bone, so the heart rate sensor stays in contact during exercise.
- Start the right workout type. A tracker counting a spin class as an outdoor ride, or strength training as "other", is using the wrong model.
- Use calorie numbers for trends and comparisons between your own workouts, not as an exact amount to eat back.
- Judge your intake by your weight trend over 2 to 3 weeks. That is the most accurate measurement you have.
Questions people ask
How accurate are fitness trackers for calories burned?
Not very. Independent studies find wrist trackers are often off by 20% or more for calories, even though most measure heart rate well. Use the number as a rough guide and for comparing your own workouts.
Which fitness tracker is most accurate for calories?
No brand is consistently accurate. In a Stanford study the Apple Watch had the lowest calorie error of seven devices, while a 2020 review found Fitbit's average closest to acceptable limits. Results vary more by activity and by person than by brand.
Do fitness trackers overestimate calories?
Sometimes. It depends on the brand and activity: reviews have found some brands mostly overestimate and others mostly underestimate. Cycling tends to be underestimated.
Should I eat back the calories my watch says I burned?
Eating all of them back can cancel a calorie deficit if the watch reads high. A cautious approach is to eat back part of them, or plan with the lower of your tracker's number and an equation-based estimate, then adjust based on your weight trend.
Sources
- Shcherbina A, Mattsson CM, Waggott D, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine. 2017;7(2):3.
- Fuller D, Colwell E, Low J, et al. Reliability and validity of commercially available wearable devices for measuring steps, energy expenditure, and heart rate: systematic review. JMIR mHealth and uHealth. 2020;8(9):e18694.
- O'Driscoll R, Turicchi J, Beaulieu K, et al. How well do activity monitors estimate energy expenditure? A systematic review and meta-analysis of the validity of current technologies. British Journal of Sports Medicine. 2020;54(6):332–340.
- Kostrna J, Oparina E, Palacios C, et al. Body fat, skin tone, and the accuracy of smartwatch caloric expenditure estimates. PLoS One. 2026;21(7):e0353261.
- Herrmann SD, Willis EA, Ainsworth BE, et al. 2024 Adult Compendium of Physical Activities. Journal of Sport and Health Science. 2024;13(1):6–12.