How Fitbit calculates calories
Fitbit starts with your basal metabolic rate, the energy your body uses at rest, worked out from the height, weight, sex and age in your profile. It adds the calories from movement detected by its sensors, and on devices with heart rate tracking it uses heart rate data, especially to estimate calories during exercise.
The calories number on a Fitbit is a total for the day that includes your resting metabolism, which Fitbit says accounts for at least half of what you burn. That is why the number climbs even while you sleep, and why it looks much higher than the active calories shown by some other devices.
How accurate is it?
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.
For Fitbit specifically: in the 2020 review of 43 studies, Fitbit was the only brand whose average fell within acceptable limits, but individual results were spread widely, underestimating in 48% of comparisons and overestimating in 40%. The 2020 meta-analysis found the Fitbit Charge HR close overall but overestimating during walking. In the 2026 study of current watches, the Fitbit Sense 2's average error looked small only after unusual readings were removed.
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.
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 is Fitbit for calories burned?
On average Fitbit has come out close in research reviews, but single workouts and single days can be well off in either direction. A 2020 review found Fitbit underestimated in 48% of comparisons and overestimated in 40%.
How does Fitbit calculate calories burned?
It adds your basal metabolic rate, estimated from your height, weight, sex and age, to the calories from activity detected by its sensors. Heart rate data is used especially to estimate calories during exercise.
Why does my Fitbit show calories burned when I haven't moved?
Fitbit's calorie count is a daily total that includes your resting metabolism, which keeps running all day and night. It accounts for at least half the calories most people burn.
Should I eat back my Fitbit calories?
Fitbit's daily total already includes your resting calories, so it is not the same as exercise calories. If you set a calorie target elsewhere, compare like with like, and judge your intake by your weight trend over a few weeks.
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.
- Google. How does my device estimate calories burned?