SpO2, ECG and Blood Pressure on Budget Watches: What the Numbers Mean

Budget wearables in India now advertise blood oxygen, ECG, blood pressure, stress and even "body temperature". These are not all the same kind of claim. Some are real measurements with known limitations; at least one is, on inexpensive hardware, essentially an estimate dressed up as a reading.

None of these features are medical devices unless the listing says the product is regulator-approved, and almost none in this price bracket are. Treat everything below as wellness information, not diagnosis.

SpO2 (blood oxygen)

What it does: shines red and infrared light into the wrist. Oxygenated and deoxygenated blood absorb the two colours differently, so the ratio estimates oxygen saturation.

How well it works: the principle is sound — it is the same as a fingertip pulse oximeter. The wrist is a considerably worse place to do it than a fingertip, because there is less blood flow and more tissue in the way. Expect readings to be in the right area but to vary by a few percentage points between attempts.

Using it sensibly: stay completely still, arm supported at heart height, wrist warm, strap snug. Movement is the main cause of wild readings. Look at repeated measurements rather than one.

ECG

What it does: measures the heart's electrical activity between two electrodes — one against your wrist, one under the finger you place on the crown or bezel. This is genuinely different from optical sensing, and much closer to what a clinic measures.

The catch: a clinical ECG uses twelve leads placed around the torso. A watch has one. A single-lead trace can show heart rhythm well — which is why the credible use is screening for atrial fibrillation — but it cannot see most of what a twelve-lead ECG is for.

What to check: whether the ECG function is actually approved by a regulator in your market. Many budget watches include an "ECG" that produces a waveform with no validation behind it. An approved feature says so explicitly and names the regulator; an unapproved one is vague.

Blood pressure

This is the one to be most sceptical about. Blood pressure is a measurement of force, and the standard way to obtain it is to squeeze the artery with a cuff until flow stops, then release. A watch with no cuff cannot do that.

What budget watches do instead is estimate, usually from pulse wave timing — how long the pressure wave takes to travel — combined with a calibration you enter from a real cuff device. The estimate drifts as your physiology changes, which is precisely when you would want it to be right.

A few genuinely cuff-based wrist devices exist and are regulated as medical devices. They are visibly different: the strap itself inflates. If the strap does not inflate and there is no approval named, the number is a model output, not a measurement.

If you need to track blood pressure, an upper-arm cuff monitor is inexpensive, standardised, and the thing a doctor will actually accept.

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Stress, body temperature and "energy"

Stress scores are derived from heart rate variability — the variation in gaps between beats. HRV is real and interesting, but a single number labelled "stress" compresses it heavily, and it responds to caffeine, illness and poor sleep as readily as to anything psychological.

Body temperature on a wrist measures skin temperature, which is not core temperature. It tracks the room, your blanket and your circulation as much as you. Useful as a night-to-night trend, useless as a fever check.

Energy, readiness and recovery scores are vendor-specific blends of sleep, HRV and activity. They are not standardised, so a score of 70 on one brand means nothing on another.

Browse health-tracking smartwatches on Amazon