Every mainstream smartwatch has a heart rate monitor. The useful question is which measurement each one is actually doing. Optical wrist sensing, ECG, background monitoring and rhythm notifications are four different capabilities that get marketed as one, and they differ in accuracy, in what they can detect, and in whether they work at all while you are moving.
This guide explains how the sensor on the back of the watch works, when its readings are trustworthy and when they are not, what the notification features do, and what to look for on a spec sheet that is actually checkable.
How a wrist heart rate sensor works
The technique is photoplethysmography. Apple explains the physics plainly: blood is red because it reflects red light and absorbs green light. The watch shines green LEDs at your skin, measures how much light comes back, and infers the pulse from the changing absorption as blood flows.
Apple Watch uses green LEDs during workouts and Breathe sessions, and to calculate walking average and heart rate variability. For background readings and heart rate notifications it switches to infrared light, which is less power-hungry. Garmin’s equivalent sensor sits in the same place, on the back of the device, in contact with the skin.
That mechanism explains every accuracy limitation that follows. If light cannot get cleanly into tissue and back out, there is no reading.
When optical readings are reliable, and when they are not
| Situation | Optical wrist accuracy | Why |
|---|---|---|
| Resting, sitting still | Good | No motion artefact, steady blood flow |
| Steady-state running or cycling | Usually good | Rhythmic, predictable motion the algorithm handles well |
| Intervals with sharp changes | Poor to fair | The sensor lags rapid rises and falls |
| Weight training, climbing, racquet sports | Poor | Wrist flexion breaks the seal and floods the sensor with motion noise |
| Cold weather | Poor | Reduced blood flow at the skin surface |
If your training is mostly the bottom three rows, a chest strap will tell you more than any watch on the market, regardless of price. That is a limitation of the measurement technique, not of a particular brand.
Fit is the single biggest accuracy variable
Both Apple and Garmin lead with fit rather than hardware. Garmin’s published guidance for erratic heart rate data is specific and worth following exactly:
- Wear the device above your wrist bone. Not on it, and not down against the hand.
- Snug but comfortable: and it should not move while running or exercising.
- Clean and dry your arm before putting the watch on.
- Avoid sunscreen, lotion and insect repellent under the device. This one causes more bad data than any other, and almost nobody knows it.
- Avoid scratching the sensor on the back of the device.
- Wait until the heart rate icon is solid before starting your activity.
- Warm up for 5 to 10 minutes and get a reading before starting. In cold environments, warm up indoors.
- Rinse the device with fresh water after each workout.
Apple’s advice starts from the same place: “Start with a good fit”, and it also notes that for a small percentage of people, various factors may make it impossible to get a reading at all. That is an honest disclosure and worth knowing before you conclude a watch is faulty.
The four capabilities to separate on a spec sheet
1. Continuous background monitoring
The watch samples your heart rate through the day without you asking. Apple Watch measures throughout the day when you are still, and periodically when you are walking, and because the sampling is tied to your activity, the time between measurements will vary. Do not expect an evenly spaced graph; the gaps are by design, not a bug.
2. High and low heart rate notifications
These alert you when your heart rate stays above or below a threshold you choose, while you appear to have been inactive for a period. You pick the beats-per-minute value yourself. This is the feature most likely to surface something you would not otherwise have noticed, and it costs nothing to enable.
3. Irregular rhythm notifications
A distinct feature that looks for an irregular rhythm rather than a rate. On Apple Watch these require watchOS 5.1.2 or later and are available only in certain countries and regions. Availability is regulatory, not technical, so a watch sold in one market may not offer what the same model offers in another. Check availability in your own country before buying for this reason.
4. ECG
A genuinely different measurement, taken by touching an electrode with the opposite hand to complete a circuit, rather than by shining light through the wrist. It is a single-lead recording, not the twelve-lead trace a clinic produces, and it is a separate feature from the optical sensor even when both appear on the same watch.
What to look for when buying
Ignore the adjectives on the box and check these:
- Can it pair an external heart rate strap? The most important spec for anyone training seriously, and the one most often omitted from consumer marketing. It lets one watch cover both use cases.
- Which regulated features are available in your country? Rhythm notification and ECG availability varies by market.
- Does background monitoring hurt battery life enough to matter? Continuous sensing is one of the largest drains on any smartwatch.
- Is the band adjustable enough to sit snugly above the wrist bone? A band that is only ever slightly loose will produce bad data forever.
- Does it export the data? If the readings only live in one app, they are much less useful over time.
For durability claims alongside the sensor specs, our guide to what MIL-STD-810 and ATM ratings actually certify applies the same principle: a named standard is checkable and an adjective is not.
A necessary caveat
Consumer smartwatches are not medical devices, and their heart rate features are not intended for medical diagnosis. Garmin directs users to its own accuracy documentation, and Apple carries the same framing on its sensing features. A notification from a watch is a prompt to speak to a clinician, never a diagnosis or a reason to ignore symptoms. If you feel unwell, the watch reading is not the relevant input.
Frequently asked questions
How accurate is a smartwatch heart rate monitor?
Good at rest and during steady-state cardio, unreliable during intervals, weight training and anything with heavy wrist movement. The sensor infers heart rate from how much green or infrared light your blood absorbs, so motion, cold and a loose fit all degrade it. For high-intensity or strength training, a chest strap remains substantially more accurate.
Why does my smartwatch show the wrong heart rate?
Most often fit. Garmin’s published guidance is to wear the device above the wrist bone, snug but comfortable so it does not move, on a clean dry arm with no sunscreen or lotion underneath, and to warm up for 5 to 10 minutes before an activity. Cold weather reduces blood flow at the skin and makes readings worse regardless of brand.
What is the difference between an optical heart rate sensor and ECG on a smartwatch?
The optical sensor shines light into the wrist and infers your pulse rate from absorption. ECG measures the heart’s electrical activity, requires you to touch an electrode with the opposite hand, and produces a single-lead recording rather than the twelve-lead trace used clinically. They answer different questions and are separate features even on watches that have both.
Does continuous heart rate monitoring drain the battery?
Yes, meaningfully. It is one of the largest ongoing power costs on any smartwatch, which is why Apple Watch uses lower-power infrared light for background readings and reserves the green LEDs for workouts. If battery life is your priority, background monitoring is usually the first thing worth turning down.
Can a smartwatch detect a heart problem?
It can flag patterns worth investigating — a rate that stays unusually high or low while you are inactive, or an irregular rhythm on watches where that feature is available in your country. It cannot diagnose anything. Consumer smartwatches are not medical devices; a notification is a reason to contact a clinician, not a conclusion.
The bottom line
Every smartwatch measures heart rate; almost none of them measure it well during the activities where people most want a number. Fit the watch properly above the wrist bone, keep lotion off your skin underneath it, warm up before you start, and buy a watch that can pair an external strap if your training is anything other than steady-state. Treat the notification features as prompts, not verdicts.
More wearables coverage on Tech2Post: what MIL-STD-810, ATM and EN 13319 actually certify, the Garmin fēnix 8’s full specs and battery figures, and Bose aviation headsets for another category where certification separates real specs from marketing.
On the audio and phone side: connecting AirPods to a laptop, finding lost AirPods, Sony’s Sound Connect app, checking iPhone data usage (fitness apps sync more than people expect), Assistive Access for a simplified iPhone, and phones built for the outdoors.
Sources: Apple: Monitor your heart rate with Apple Watch for the photoplethysmography explanation, green LED versus infrared usage, background measurement behaviour, heart rate notifications and the irregular rhythm notification requirements; Garmin: Tips for Erratic Heart Rate Data for the fit, skin preparation and warm-up guidance; and Garmin: Wearing the Device for the wrist-bone positioning guidance.
