Modern smartwatches and fitness rings pack a surprising number of sensors. Here’s what each one measures and how much to trust it.

Optical heart rate (PPG)

Nearly every wearable uses an optical sensor that shines green LEDs into your skin and measures how the light scatters as blood pulses — this is photoplethysmography (PPG). It’s good for continuous heart-rate trends and workout zones, though it can lag during rapid interval changes and is less accurate than a chest strap.

Blood oxygen (SpO2)

Using red and infrared light, the same style of sensor estimates blood-oxygen saturation. It’s useful for spotting trends (and sleep patterns), but wrist-based SpO2 is a wellness estimate, not a medical-grade reading.

ECG (electrocardiogram)

Some watches add electrodes that take a single-lead ECG when you touch the crown or bezel. This can flag irregular rhythms like atrial fibrillation — a genuinely useful health feature, though it’s a screening tool, not a diagnosis.

GPS

Built-in GPS lets a watch track pace, distance, and route without your phone. “Multi-band” or “dual-frequency” GPS is more accurate in cities and under tree cover. If a watch lacks its own GPS, it borrows your phone’s — fine for casual use, less so for phone-free runs.

Others you’ll see

Accelerometers and gyroscopes count steps and detect falls; a barometer/altimeter tracks stairs and elevation; skin-temperature sensors add sleep and cycle insights.

Bottom line: wrist sensors are great for trends and motivation, and ECG can be a real safety net — but treat the numbers as wellness estimates, not clinical measurements. Prioritize the sensors that match your goals (GPS for runners, ECG for heart-health peace of mind).

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