An ECG, or electrocardiogram, measures the heart's electrical activity, and on a smartwatch the mechanics are simple: a sensor in the caseback pairs with the finger you press to the crown or side button, closing a circuit that records a roughly 30-second trace. This is not a clinical 12-lead test, it is a single channel similar to a Lead I ECG, but the purpose is clear: to flag irregularities that point to atrial fibrillation (AFib) and similar rhythm issues. The feature arrived in 2018 on the Apple Watch Series 4, which earned FDA De Novo classification as an over-the-counter ECG device, and the category has widened quickly since.
Today ECG lives across several ecosystems, each with quirks worth knowing. On the Apple side, every numbered model from Series 4 through Series 10 plus the Apple Watch Ultra line carries the ECG app, with the Series 10 starting at around £399. Samsung's Galaxy Watch family, from the Active2 up to the roughly £520 Galaxy Watch Ultra, includes ECG, but with a real catch: it only works when the watch is paired with a Samsung Galaxy phone, not an iPhone or another brand of Android. In the hybrid camp, the Withings ScanWatch 2 at around £319 blends a classic analogue face with a two-channel recording, and Garmin's Venu 3 and Fenix 8, the Fitbit Sense 2, and every Google Pixel Watch generation round out the field.
Choosing one starts with your phone. iPhone owners are well served by the Apple Watch or the ScanWatch 2, while a Galaxy phone unlocks Samsung's full ECG function. After that, weigh battery life, looks, and how much data the trace captures. The ScanWatch 2's two-channel approach records more than the single lead of most rivals, giving a cardiologist a slightly richer picture, and its near-30-day battery and traditional hands appeal to anyone who dislikes the smartwatch look. Apple and Samsung counter with deeper notification and app ecosystems.
One nuance trips up buyers: ECG is not standard across a whole product line. The cheaper Withings ScanWatch Light at around £229, for instance, drops ECG, SpO2, the altimeter and skin temperature entirely, so confirm the exact model and its authorised FDA or CE clearance before paying. And keep the limits in view. These watches screen, they do not diagnose; an irregular trace is a prompt to see a physician, not a verdict from the wrist.