Apple Watch heart rate is accurate at rest and steady effort

The Apple Watch reads your heart rate within about 2 to 5 beats at rest and during steady running. It gets less reliable during fast intervals or anything with a lot of wrist motion.

Why it works

The watch uses photoplethysmography (PPG), shining light into the wrist to read how blood volume shifts with each beat. Green LEDs handle continuous readings during exercise. Infrared sensors take over for resting measurements. Wrist movement scatters the light and can corrupt the signal, which is why fast, jerky activity throws it off.

The evidence

• Lambe, R., Baldwin, M., O'Grady, B. et al. (2026). The Accuracy of Apple Watch Measurements: A Living Systematic Review and Meta-Analysis. npj Digital Medicine.

  This living review pooled 82 studies covering 430,052 participants and 14 health metrics measured by Apple Watch through Series 9 and Ultra 2. Heart rate was accurate overall (mean bias -0.27 bpm), and during exercise 10 of 11 studies (91%) reported a mean absolute percentage error under 10%. Energy expenditure was the weakest metric: every study that calculated it reported an error of 20% or higher in at least one condition, with mean absolute percentage error ranging from about 9.7% (running) to 151.7% (walking). Blood oxygen bias was near zero (-0.04%) but with wide agreement limits, and atrial-fibrillation detection had 79% sensitivity and 91% specificity, good enough for screening but not standalone diagnosis.

• O'Grady, B., Lambe, R., Baldwin, M. et al. (2024). The Validity of Apple Watch Series 9 and Ultra 2 for Serial Measurements of Heart Rate Variability and Resting Heart Rate. Sensors.

  In 39 healthy adults who gave 316 measurements over 14 days, Apple Watch Series 9 and Ultra 2 heart-rate-variability (HRV) readings were compared against a Polar H10 chest strap (the reference). The watch underestimated HRV by an average of 8.31 ms (p = 0.025), with a mean absolute percentage error of 28.88% and a mean absolute error of 20.46 ms, which the authors judged below clinical accuracy standards. Resting heart rate, by contrast, was relatively accurate. The practical read: the watch is useful for tracking day-to-day trends in HRV but not for trusting exact absolute values.

• Falter, M., Budts, W., Goetschalckx, K. et al. (2019). Accuracy of Apple Watch Measurements for Heart Rate and Energy Expenditure in Patients with Cardiovascular Disease: Cross-Sectional Study. JMIR mHealth and uHealth.

  In 40 patients with cardiovascular disease cycling on an ergometer, Apple Watch readings were compared against a 12-lead ECG (for heart rate) and indirect calorimetry (for energy expenditure). Heart-rate agreement improved with intensity: intraclass correlation was 0.729 at rest, 0.828 at moderate effort, and 0.958 at peak, judged clinically acceptable. Energy expenditure correlated reasonably (intraclass correlation 0.797) but was systematically overestimated, averaging about 30.5 kcal too high per test with a mean absolute percentage error of 114.72%. The authors concluded the watch may be usable for heart-rate-guided training but should not be relied on for calorie estimates.

Why we call confidence high

A 2026 living systematic review and meta-analysis by Lambe et al. confirms Apple Watch heart rate accuracy across several generations. A separate validation of the Series 9 and Ultra 2 (O'Grady et al. 2024) found strong agreement with reference devices for resting heart rate (RHR) and heart rate variability (HRV).

Where it applies

Healthy adults