Sports watch: what it actually measures

A sports watch throws dozens of numbers at you every day, but only a few of them come straight from a sensor — the rest are estimates produced by an algorithm. That difference matters: some numbers are good enough to plan a session around, others only make sense as a trend across several weeks. Here is what is actually measured, how big the error is, and how to test your own watch.

The sensors: what physically exists in there

An average sports watch has four to six sensors:

• Accelerometer and gyroscope — acceleration and rotation of your arm. Steps, cadence and part of sleep detection come from here.
• Optical heart rate sensor (green light) — measures changes in how light reflects off blood in the capillaries of your wrist.
• GPS/GNSS — position, and from that distance and pace.
• Red and infrared diodes — an estimate of blood oxygen saturation (SpO2).
• Thermometer — skin temperature, not body temperature.

Everything else — calories, VO2max, sleep stages, recovery score, stress level, training load — is not measured but calculated from those inputs plus your height, weight, age and sex.

Heart rate: the most useful number, with one limit

At rest and during steady effort the optical sensor is good: the error is usually 2–5 beats per minute compared to ECG. The problem starts when intensity changes fast. During intervals, sprints, lifting, and anything where the wrist bends or grips a bar, the error grows to 10–30 beats and the watch lags 10–20 seconds behind your real heart rate. On 30-second intervals that means you only see the peak once you are already resting.

Two things fix accuracy immediately: wear the watch 1–2 cm above the wrist bone and tighten it enough that it does not slide when you shake your arm, while still leaving room for a finger under the strap. If you train intervals or strength and the number genuinely matters, a chest strap reads the heart's electrical signal and its error is under 2 beats.

A five-minute test

1. Sit still for 3 minutes and write down your resting heart rate.
2. Run or walk fast at a steady pace for 10 minutes and check the value at the end.
3. Do 4×30 seconds hard with 60 seconds rest, and watch how the reading behaves.

If in the third part the number jumps around with no logic, or sticks at 130 while you are gasping, your watch is a rough guide during intervals, not a measuring instrument.

Steps, distance and pace

Step counting is off by 5–10% per day; slow walking around the house, pushing a cart or carrying something makes it worse. For GPS distance, expect 1–3% deviation in the open and 5–10% in a dense city or under trees. The check is simple: run 4 laps in lane one of a running track, which is 1600 m — the watch should show between 1570 and 1630 m. Instantaneous pace swings the most, so judge your effort by the per-kilometre average, not a number that changes every second.

Calories: the number with the biggest error

Being blunt about this one: calorie burn is an estimate built from heart rate, movement and your body weight, and the error is typically 20–30%, and over 50% for some activities. If the watch says 600 kcal for a 45-minute run, the real figure is probably 420–550. Strength training is worse still, because the watch has no idea how many kilograms you lifted or how many sets you did.

Practical rule: do not eat back the calories the watch gives you. If you are running a deficit, build it from food and the scale — 0.3–0.5 kg per week is a realistic rate. Use the watch to compare yourself to yourself: was today's session harder than last week's.

Sleep, HRV and recovery scores

Total sleep time is estimated decently, usually within 20–30 minutes. Sleep stages are another story — agreement with lab measurement is roughly 50–70%, so 45 minutes of deep sleep should not be taken literally. The trend is still useful: if you sleep 5 hours instead of 7 for five nights in a row, the watch sees that perfectly well.

HRV (heart rate variability) and resting heart rate are the most useful recovery signals, but only as a line over time. Daily HRV values swing 10–20 ms for no reason at all. Use a 7-day average as your baseline; if your morning resting heart rate is 5–7 beats higher three mornings in a row, or HRV sits clearly below your average, that is a good reason to swap a hard session for an easy 30–40 minutes.

VO2max, SpO2 and other estimates

Watch VO2max is calculated from the pace-to-heart-rate relationship and is off by 10–15% compared to a mask test. The number itself means little; a change of 2–3 units over 8–12 weeks means something. Wrist SpO2 deviates by 2–4% and is very sensitive to arm movement and cold fingers — a single 93% reading in the middle of the night is not a diagnosis. Stress level is mostly HRV under a different name, and the various battery metrics combine sleep and the last few days of load.

When to see a doctor

A watch is not a medical device, but it can point at something worth checking:

• Resting heart rate consistently above 100, or below 40 if you are not a trained endurance athlete.
• Repeated irregular rhythm alerts, especially with palpitations, dizziness or fainting.
• Chest pain or pressure, or breathlessness at an effort you used to handle easily.
• SpO2 below 90% across several readings while you are awake and still.
• Heart rate staying 20+ beats above normal for days along with fatigue and fever.

Bring the watch data to the appointment, but do not wait for the watch to confirm a symptom you can already feel.

The short version

• Sensors measure acceleration, reflected light and position; calories, sleep stages and VO2max are calculations, not measurements.
• Heart rate is accurate to 2–5 beats in steady effort but off by 10–30 during intervals and lifting — a chest strap fixes that.
• Calories are off by 20–30%, so plan nutrition from food and the scale instead.
• Read HRV and morning heart rate as a 7-day average, never as a single day's number.
• Chest pain, fainting or repeated rhythm alerts mean a doctor, whatever the watch says.