How accurate are Apple Watch calories for HIIT, CrossFit and weightlifting?

Treat them as a rough guide. A 2025 University of Mississippi review of 56 studies found Apple Watch calorie estimates had an average error of about 28%, compared with about 4% for heart rate and 8% for steps. The error showed up across walking, running, cycling and mixed-intensity workouts. Heart rate is the more reliable number.

How it works

What the research says

Researchers at the University of Mississippi reviewed 56 studies that compared Apple Watch measurements with reference tools. The mean absolute percentage errors were 4.43% for heart rate, 8.17% for step count and 27.96% for energy expenditure. The calorie error held across walking, running, cycling and mixed-intensity workouts (University of Mississippi, June 2025).

For weightlifting, a 2026 lab study had 62 men do a resistance session (bench press, squats, T-bar rows and deadlifts) with an Apple Watch Series 7. The watch reported 299 kcal on average; indirect calorimetry measured 141 kcal. Its heart rate, though, matched an ECG closely (Lee et al., 2026). We haven’t found a study that tests Apple Watch calories in a mixed HIIT or CrossFit-style workout, so there’s no reliable figure for those.

How Apple Watch estimates calories

The watch combines its sensors with your personal information: Apple says it uses your height, weight, gender and age (Apple: Calibrate your Apple Watch). Since the estimate depends on them, keep those details up to date in the Health app.

Calories in Atlon

Atlon doesn’t calculate calories itself. It shows the active calories Apple Health measures during the workout, and saves the workout to Apple Health with them. So Atlon’s calories have the same accuracy as Apple’s.

Atlon results on Apple Watch: calories next to Work, Effort and Form, with average and max heart rate below

Limits

Updated