Wearables · Devices
Apple Watch: what it measures and its limits
A factual overview of Apple Watch health features: what the sensors measure, what is estimated, which models support ECG and Blood Oxygen, and the limitations.
Written by LON6EVITY Editorial Team · Last updated
Measured with sensors
Apple Watch measures heart rate with an optical sensor and movement with motion sensors. Apple's support pages describe heart rate monitoring throughout the day and high and low heart rate notifications on Series 1 or later.
ECG and heart rhythm
According to Apple, the ECG app uses the electrical heart sensor on Apple Watch Series 4 or later and all Ultra models to record a single-lead ECG and check for signs of atrial fibrillation. Irregular rhythm notifications use the optical sensor. Both are available only in certain countries, are not intended for people under 22, and are not designed for people already diagnosed with AFib.
Blood oxygen
Apple states the Blood Oxygen app works on Series 6 or later and all Ultra models, is available only in certain regions, and is designed for general fitness and wellness, not medical use. For some US models purchased from 18 January 2024, the analysis is performed on the paired iPhone.
Estimated, not measured
Metrics such as cardio fitness (estimated VO2 max), sleep stages and calories are algorithmic estimates.
Limitations
Feature availability depends on model, software version and region. The watch does not detect heart attacks and is not a substitute for a clinical assessment.
Factors that can affect the result
- Model generation
- Region
- watchOS version
- Fit on the wrist
Questions people commonly ask
- Can Apple Watch detect a heart attack?
- No. Apple states that the ECG app cannot detect heart attacks.
- Does every Apple Watch have ECG?
- No. It requires Series 4 or later or an Ultra model, and availability depends on country or region.
Related tests
Related topics
Bring your health data together
What does this mean for your own health data? Log your results, follow trends over time and bring an organised summary to your next doctor's appointment.
Sources
- Take an ECG with the ECG app on Apple Watch. Apple Support.
- Heart health notifications on your Apple Watch. Apple Support.
- How to use the Blood Oxygen app on Apple Watch. Apple Support.
- Monitor your heart rate with Apple Watch. Apple Support.