Deficitly
Open the calculator
Are fitness trackers accurate for calories?
Not very. Wrist trackers measure steps and heart rate fairly well, but the calories they report were off by roughly 27% to 93% depending on the device, according to the best-known test (Stanford, 2017). They guess calories from your movement, heart rate and the weight and age you entered. Treat the number as a rough guide, not a measurement. For a woman of 70 kg (154 lb) who walks 8,000 steps and lifts three times a week, the engine says about 1,970 calories a day in total, of which only about 270 are active.
What do studies say about tracker accuracy?
- Steps: usually close for ordinary walking, and less accurate for slow walking, pushing a pram or cycling. Reviews of consumer trackers find step counts the most dependable measure (Evenson et al., 2015).
- Heart rate: good at rest and in steady effort; worse in intervals, weights and in cold or on darker or tattooed skin.
- Energy expenditure (calories): the weak point. In the Stanford study (Shcherbina et al., 2017) seven popular wrist trackers were tested on 60 people. Heart rate errors were small for most devices, but none was within 20% on calories, and the errors ranged from about 27% to 93% depending on the device and activity. A 2020 systematic review (Fuller et al.) reached the same view: steps and heart rate acceptable, energy expenditure not.
Errors run both ways, so a tracker can't tell you if you're "behind". That's why the calorie figure should not set your daily food (how accurate are TDEE calculators? has the same warning for formulas).
Active calories vs total calories: why trackers disagree with calculators
Most confusion is about what the number means:
- Active calories are only what movement and exercise burn above rest. The Apple Watch "Move" ring counts active calories only.
- Total calories add your resting burn and the rest of the day. Garmin and Fitbit headline figures are usually the total.
| Part of the day | Calories | In a tracker |
|---|---|---|
| Resting burn (BMR) | 1,370 | Total |
| Daily life (base NEAT) | 140 | Total |
| Digestion (TEF) | 200 | Total |
| Steps | 190 | Active and total |
| Workouts | 80 | Active and total |
| Total | 1,970 | Garmin / Fitbit style |
| Active only | 270 | Apple Watch Move style |
So an Apple Watch saying "250" and a calculator saying "1,950" aren't in conflict; they measure different things. More on the parts of a day: how many calories you burn in a day and what is NEAT?.
Is the Apple Watch accurate for calories?
It has been among the better devices in studies, but it's still an estimate. The Stanford study's summary names it the device with the lowest overall error for both heart rate and calories (by median calorie error per task, the Fitbit Surge did slightly better), but like every device it missed calories by more than 20%. Its resting and active split is useful; its total depends on the weight, height and age you entered, and a stale weight in Health skews every number. Fix your profile first. The same applies to Garmin, Fitbit, Whoop and Oura: use the trend, not the day.
How should you use tracker data?
- Trust your steps. Feed your average (a week or two) into the calculator above or the maintenance calorie calculator instead of the tracker's calorie number (what 10,000 steps burn).
- Don't eat back all your exercise calories. If you do, eat back at most half, and only after a few weeks of results (should you eat back exercise calories?).
- Check against your scale. Over 3–4 weeks your weight trend and food log give your real burn: the observed calculator in how to find your real TDEE does it, and signs of a fast or slow metabolism explains what to expect.
- Use it for what it's good at: steps, a nudge to move, heart rate trends and consistency.
Which tracker is most accurate for calories?
None well enough to bet your diet on. Chest straps are better for heart rate; a power meter is better for cycling work; but no consumer device gives a trustworthy calorie figure for a whole day. What matters more is consistency: use the same device and compare your own numbers over time.
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. J Pers Med 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 Uhealth 2020;8(9):e18694.
- Evenson KR, Goto MM, Furberg RD. Systematic review of the validity and reliability of consumer-wearable activity trackers. Int J Behav Nutr Phys Act 2015;12:159.