The sensor stack, bottom to top
PPG — light through skin
Green LEDs flash into your wrist ~100×/sec; a photodiode measures reflected light. Blood absorbs green light, so each pulse dims the reflection — that rhythm is your heart rate. Accuracy at rest: within ~2–3 bpm of an ECG. During intervals or with wrist tattoos/motion, error grows; chest straps still win for hard efforts.
Accelerometer — motion in 3 axes
A microscopic mass on springs measures acceleration thousands of times a second. Step counting is peak-detection on that wave; modern algorithms are ~95% accurate walking, worse pushing a stroller or shopping cart (your wrist stops swinging).
GPS — trilateration
Your position from satellite time-of-flight, typically within 3–5 m in the open, worse in cities and under tree cover. Dual-frequency chips in newer flagships cut urban error roughly in half — a big reason review rankings shuffle year to year.
Temperature & HRV
Overnight skin temperature and heart-rate variability (beat-to-beat gaps) power readiness and cycle tracking. These are trend instruments: a 0.4 °C deviation matters versus your baseline, not anyone else's.
The number to trust least: calories
Calorie burn is estimated from heart rate, motion, and your profile — validation studies put typical wrist-tracker energy-expenditure error at 15–30%. Worked example: a display reading of 600 kcal for an hour's ride could plausibly be anywhere from ~450 to ~750 kcal.
Practical rule: use calories for week-over-week trends (same device, same you), heart rate for training zones, and GPS distance for pacing. That's also how serious reviewers test — against ECG straps and calibrated metabolic carts, not against vibes.
Choosing a tracker in 2026
Match the sensor to your sport: runners want dual-frequency GPS and a reliable wrist HR at tempo; lifters mostly need good manual logging (accelerometers can't see a squat); sleep-focused buyers should prioritize temperature sensing and validated staging. The best tracker is the one whose numbers you'll actually check weekly.