The ECG on Apple Watch is a single-lead electrocardiogram that records the electrical activity of your heart for 30 seconds. Available on Series 4 and later (including all Ultra models), it uses built-in electrodes to capture a rhythm strip similar to one view of a hospital ECG, then classifies your heart rhythm as normal, atrial fibrillation, or inconclusive. The feature received FDA clearance as a Class II medical device, making it the first direct-to-consumer ECG available on a smartwatch.
How the Hardware Works
The Apple Watch has electrodes in two places: the back crystal that sits against your wrist and the Digital Crown on the side. When you open the ECG app and rest your finger on the Digital Crown, you complete an electrical circuit between both arms and across your chest. This lets the watch pick up the tiny electrical impulses your heart generates each time it beats.
That closed circuit mimics what happens with Lead I on a standard clinical ECG, which measures the voltage difference between your right arm and left arm. The watch records 30 seconds of this signal, plots it as a waveform, and runs an algorithm to classify the rhythm. The entire process happens on your wrist with no additional equipment.
What It Measures (and What It Can’t)
The ECG app is designed to do one thing well: detect atrial fibrillation, the most common serious heart rhythm disorder. AFib causes the upper chambers of the heart to beat irregularly, raising the risk of stroke and other complications. In Apple’s internal validation study of 588 people, the algorithm showed greater than 98% sensitivity and greater than 99% specificity for classifying rhythms as normal sinus rhythm or AFib.
Independent research published in Circulation by the American Heart Association found that when clinicians reviewed the actual PDF waveform from the watch (rather than relying only on the on-screen notification), sensitivity for detecting AFib reached 96% with 100% specificity. The on-screen notification alone was less reliable, catching only 41% of AFib episodes, largely because many readings came back as inconclusive rather than wrong.
The ECG app explicitly cannot detect heart attacks, blood clots, strokes, high blood pressure, congestive heart failure, high cholesterol, or other types of arrhythmia beyond AFib. If the watch gives you a “sinus rhythm” result, that means the rhythm looks normal. It does not mean your heart is healthy in every other respect.
Single-Lead vs. Hospital ECG
A standard hospital ECG uses 10 electrodes placed across your chest and limbs to generate 12 different electrical views of the heart. Each view reveals activity in a different region, which is why a 12-lead ECG can identify heart attacks, pinpoint which artery is blocked, and detect a wide range of structural problems. The Apple Watch captures just one of those 12 views.
That single lead is still surprisingly informative for rhythm analysis. A study in the Journal of the American College of Cardiology compared the Apple Watch waveform directly against a clinical 12-lead and found 100% agreement on rhythm interpretation. Heart rate measurements differed by only about 1 beat per minute on average. Other intervals on the tracing (the timing of different phases of each heartbeat) showed moderate to strong agreement, though with enough variation that a cardiologist wouldn’t use the watch recording alone for detailed waveform analysis.
In practical terms, the Apple Watch ECG is a screening tool, not a diagnostic one. It’s useful for catching irregular rhythms in everyday life, especially because AFib often comes and goes. Being able to capture a 30-second strip the moment you feel palpitations gives your doctor real data from that exact moment, something even a scheduled office visit can’t always provide.
How to Take a Reading
Open the ECG app on your watch, rest your arm on a flat surface like a table or your lap, and hold your finger gently on the Digital Crown. Stay still for 30 seconds. Movement, loose band fit, or a wet wrist can interfere with the reading and produce an inconclusive result. The app will display your heart rhythm classification and heart rate when the recording finishes.
Each recording is saved automatically to the Health app on your iPhone. From there, you can tap into your ECG history, select any individual reading, and export it as a PDF. That PDF shows the full 30-second waveform along with the classification result, and you can share it directly with your doctor through email, a patient portal, or messaging. Many cardiologists now accept these PDFs as a useful supplement to in-office testing, particularly for patients who report intermittent symptoms.
Who Can Use It
The FDA clearance specifies that the ECG app is not intended for people under 22 years old. Apple’s algorithm was validated on adult populations, and heart rhythm characteristics differ enough in younger people that the classifications may not be reliable. The app is also not designed for people who already have a known AFib diagnosis, since it’s built as a screening tool for detecting previously unrecognized AFib rather than monitoring a known condition.
You’ll need an Apple Watch Series 4 or newer, or any Apple Watch Ultra model. The Apple Watch SE does not include the electrical heart sensor required for ECG readings, though it does have a basic optical heart rate sensor. Your paired iPhone also needs to be running a compatible version of iOS, and the ECG feature must be available in your country, as regulatory approval varies by region.
When the Results Say “Inconclusive”
Inconclusive readings are common and don’t necessarily mean something is wrong. A heart rate below 50 or above 150 beats per minute can trigger an inconclusive result, as can movement during the recording, a poor contact between your finger and the Digital Crown, or certain arrhythmias the algorithm wasn’t trained to classify. If you consistently get inconclusive results, try tightening the band slightly, making sure your wrist is dry, and sitting completely still with your arm supported.
The gap between the algorithm’s on-screen classification and what a trained clinician can see on the exported waveform is worth noting. In the Circulation study, the algorithm flagged only 41% of AFib cases through its notification, but when doctors reviewed the raw PDF tracings from those same recordings, they identified 96% of AFib cases. This means an inconclusive or even “sinus rhythm” result doesn’t guarantee your rhythm was normal. If you’re experiencing symptoms like a racing heart, fluttering, dizziness, or shortness of breath, the exported waveform itself may contain information your doctor can interpret even when the algorithm couldn’t.