One letter apart, and about a thousand times different in size.

EEG is electroencephalography — the brain. ECG, also written EKG from the German Elektrokardiogramm, is electrocardiography — the heart. Both record voltages from electrodes on the skin, and there the similarity ends.

Side by side

Source. ECG records one muscle, contracting as a coordinated unit on a regular cycle. EEG records the summed activity of an enormous number of cortical neurons that are not doing any single thing.

Amplitude. This is the difference that governs everything else. A QRS complex at the chest is on the order of a millivolt. Scalp EEG is on the order of tens of microvolts — roughly a hundredth as large, and in some comparisons a thousandth. Almost every practical difficulty in EEG follows from working that close to the noise.

Shape. An ECG is a repeating stereotyped waveform. You can look at a single beat and say whether it is normal. EEG has no equivalent unit — it is continuous, irregular, and interpreted statistically or by rhythm rather than beat by beat.

Placement. ECG electrodes go on the torso and limbs, and the standard 12-lead arrangement has been stable for decades. EEG electrodes go on the head, most commonly to the international 10–20 system, and where they sit changes what you see far more dramatically.

What it is used for. ECG is diagnostic for rhythm and conduction problems and part of routine cardiac care. Clinical EEG is central to epilepsy and sleep medicine and is not, on its own, a general diagnostic test of brain health.

Why the heart appears in EEG

The heart's electrical field is large enough to be detectable across the whole body, including at the scalp. In EEG this shows up as the cardiac field artefact — a small, regular blip synchronised to the heartbeat, most visible when an electrode happens to sit over a vessel or when the reference is placed on the ear or mastoid.

It is usually small enough to ignore and occasionally large enough to be mistaken for a rhythm. Related and more troublesome is the pulse artefact, where an electrode physically moves a fraction of a millimetre with each pulse and generates a signal from the movement itself rather than from any electrical field.

The traffic goes one way. The brain's field is far too small to contaminate an ECG.

Which one wearables usually have

Most consumer wearables measure neither. A wrist or ring device typically uses PPG — photoplethysmography — which shines light into the skin and measures the change in reflection as blood volume varies with each beat. That yields heart rate and, with more processing, heart rate variability.

PPG is not ECG. It is measuring an optical consequence of the heartbeat rather than the electrical event, it is more susceptible to movement, and the beat-to-beat timing it produces is less precise — which matters, because HRV is entirely about beat-to-beat timing. Some smartwatches add a genuine single-lead ECG through a finger contact, and that is a different and better measurement, taken deliberately rather than continuously.

Why this distinction is worth keeping straight

Because the words "measures your heart" and "measures your brain" imply completely different things about what a device can say, and the acronyms are close enough that the difference gets lost in a product description. A device reading the heart can infer things about sleep and stress. A device reading the brain measures some of them more directly and is much harder to get right outside a controlled setting.