Not very accurate. A 2025 review of 56 studies found Apple Watch calorie estimates off by about 28% on average, against about 4% for heart rate. In a 2026 lab study of strength exercise, the watch reported 299 calories on average for a session that measured 141 by indirect calorimetry. Heart rate stayed accurate throughout. Use calories to compare your own similar workouts, and heart rate to judge how hard you worked.
How accurate are Apple Watch calories in general?
Off by about a quarter on average. Minsoo Kang and Ju-Pil Choe at the University of Mississippi reviewed 56 studies that compared the Apple Watch with trusted reference tools. The mean absolute percentage error was 4.43% for heart rate, 8.17% for step count and 27.96% for energy expenditure (University of Mississippi, 2025).
An average error of 28% doesn’t mean every workout is 28% out. Some estimates land close, others further off, and you can’t tell from one workout which kind it was.
Are calories less accurate in strength and HIIT workouts?
For strength work, yes, by a lot. Lee and colleagues tested four smartwatches, including the Apple Watch Series 7, on 62 healthy men, with indirect calorimetry as the reference. In a 28-minute endurance session on a treadmill, the Apple Watch reported 233 kcal on average against 204 kcal measured. In a 34-minute resistance session (two sets each of bench press, squats, T-bar rows and deadlifts at a 10-rep max), it reported 299 kcal against 141 kcal measured: about twice as much (Lee et al., 2026).
The authors conclude that “smartwatch-derived EE estimates do not accurately reflect the metabolic demands, particularly during resistance exercises”, and that watch calories “may be more appropriately used to assess relative changes in activity or overall movement trends rather than as absolute indicators of energy expenditure” (Lee et al., 2026).
For mixed HIIT and functional fitness workouts, we haven’t found a study that measures Apple Watch calories against a lab reference. Those workouts combine cardio, where the watch does better, with lifting, where it does worse, and short bursts with rests. There’s no good reason to expect them to be more accurate than either.
Why is heart rate accurate when calories aren’t?
Because heart rate is measured and calories are estimated. The watch’s sensor reads your pulse directly, and in Lee and colleagues’ study it agreed closely with an ECG in both sessions. Calories are a calculation on top of the sensors, using your personal details: Apple says the watch uses your height, weight, gender and age (Apple: Calibrate your Apple Watch). Every step of that calculation adds room for error, and the studies show it.
What should I track instead?
Track what the watch measures well, and what the workout was actually made of.
- Heart rate for how hard you worked. Average and max heart rate are the most reliable numbers the watch gives you.
- The work itself: reps, rounds, sets and time. A workout you can repeat and compare tells you more about your fitness than a calorie estimate.
- Calories only between similar workouts. The same AMRAP, done twice a month apart, gives calorie numbers that are at least comparable with each other.
Keep your height, weight and age up to date in the Health app, because the estimate depends on them.
How does Atlon handle calories?
Atlon doesn’t calculate calories itself. It shows the active calories Apple Health measures during the workout, the same estimate Apple’s Workout app uses, and saves the workout to Apple Health with them, so it counts towards your Activity rings. Atlon and Apple Health
Next to calories, Atlon shows what the watch measures well. It recognises each exercise, counts your reps and rounds, and splits the workout into work and rest. Its Effort score is built on your heart rate, and your results show your average and max heart rate. How does Atlon measure effort?
For everything else about tracking a WOD on your watch, from workout types to timers, see the complete guide to tracking WODs and HIIT on Apple Watch. For a short answer, see how accurate are Apple Watch calories for HIIT?
Get Atlon free on the App Store
Sources
- University of Mississippi. Study examines how well wearable tech tracks fitness metrics, 2 June 2025. On Minsoo Kang and Ju-Pil Choe’s review of 56 studies.
- Lee TH, Jun DU, Bae JY, Roh HT, Cho SY. Comparative validity of smartwatch-derived heart rate and energy expenditure during endurance and resistance exercise. Sensors, 2026;26(8):2526.
- Apple Support. Calibrate your Apple Watch.