Short answer
For heart rate during exercise, a chest strap came closest to an ECG in the studies below; a good smartwatch is close at rest and in steady cardio but drifts during intervals, cycling, weights and arm-heavy workouts. In a Cleveland Clinic study of 50 adults, a chest strap agreed with an ECG almost perfectly (concordance 0.996), while four wrist devices ranged from 0.67 to 0.92, with errors of up to ±34 beats per minute depending on the activity (1).
For a resting reading, all three tools can work. A meta-analysis of smartphone camera apps found an average difference of −0.32 bpm from reference devices in adults sitting still with a normal rhythm (2). For a morning HRV check, a phone camera and a chest strap both agreed closely with ECG in a study of 29 people (3).
So the real question isn't "which device is best" but "what job do you need it for". If your watch said 175 when you felt like 140, you're not imagining it: optical sensors genuinely struggle with some movements, and that's a property of the technology, not a sign you're doing something wrong.
Note: this article is for general education and is not medical advice. None of these consumer devices diagnose heart conditions, and an optical reading is not an ECG. If you have chest pain or pressure, fainting, severe shortness of breath, or a fast or irregular heartbeat with lightheadedness, call 911 (9). Device features were checked on manufacturers' pages as of November 2026. This guide is published by Welltory, which makes one of the apps mentioned.
Why the three tools give different numbers
They don't measure the same thing.
A chest strap reads electricity. Its electrodes sit on your skin near the heart and pick up the electrical signal of each beat, the same signal an ECG records. Each beat produces a sharp spike, so the strap can time beats to the millisecond. That's why researchers often use a strap as the reference when they test other devices.
A watch or ring reads light. On the back of the device, green or infrared LEDs shine into the skin, and a sensor measures how much light comes back. Each heartbeat pushes a little more blood into the tiny vessels under the sensor, and the reflected light changes. This method is called photoplethysmography, or PPG. It measures the pulse wave arriving at your wrist, not the heartbeat itself. It works well when you're still, but anything that moves blood or shifts the sensor — swinging arms, gripping handlebars, a loose band, cold skin — adds noise.
A phone camera also reads light, using the flash and the camera lens on your fingertip (5). Because your finger is pressed directly over the lens and you hold still, the signal is often clean at rest. But it's a spot check: you can't run, lift or sleep with a finger on your phone.
In short: electrodes are robust to movement; light is sensitive to it. Most of what follows comes from that one difference.
What the studies actually found
The research on this question is consistent, and it's worth knowing what was tested rather than relying on marketing.
| Study | Who and what | What it found |
|---|---|---|
| Wang et al., Cleveland Clinic, JAMA Cardiology (4) | 50 healthy adults on a treadmill; Apple Watch, Fitbit Charge HR, Basis Peak and Mio Alpha vs a Polar H7 chest strap checked against ECG | Wrist accuracy was best at rest and fell with exercise; 95% of differences were roughly −27 to +29 bpm for the best devices and wider for others; "none achieved the accuracy of a chest strap" |
| Gillinov et al., Cleveland Clinic (1) | 50 adults on treadmill, bike and elliptical; four wrist devices vs ECG | Chest strap agreement 0.996; wrist devices 0.67–0.92; worst on the elliptical at high intensity and with arm levers |
| Bent et al., Duke, npj Digital Medicine (6) | 53 people across all six Fitzpatrick skin tones; six wrist devices vs ECG | Error during activity was on average about 30% higher than at rest; skin tone was not a significant driver of error; walking and typing raised error in most devices |
| Gilgen-Ammann et al., European Journal of Applied Physiology (7) | 10 people, five activities, Polar H10 vs a medical ECG Holter | The strap's signal quality was 99.6% overall and 99.4% at high intensity, better than the Holter; correlation 0.997 |
| De Ridder et al., JMIR Cardio meta-analysis (2) | 14 studies of phone camera heart rate apps | In adults at rest with a normal rhythm, mean difference −0.32 bpm; agreement depended on keeping the finger still with steady pressure; not validated in children |
| Plews et al., International Journal of Sports Physiology and Performance (3) | 29 healthy people at rest; phone camera PPG and Polar H7 vs ECG for HRV (rMSSD) | Both agreed with ECG; differences were "trivial" |
Two things stand out. First, every study found that wrist accuracy depends on what you're doing, not just on which brand you wear. Second, the resting results are reassuring for all three methods. Most of the disagreement people notice happens in workouts.
A caution on the age of these studies: devices and algorithms have improved since 2016–2020, and newer watches may do better. But the physics hasn't changed. Light-based sensors still have to separate the pulse from motion, and chest electrodes still don't.
Which tool to use for each job
| What you want to know | Best tool | Good enough | Why |
|---|---|---|---|
| Heart rate zones in steady running or walking | Chest strap | Smartwatch, snug fit | Rhythmic motion is the easiest case for wrist sensors |
| Intervals, sprints, HIIT | Chest strap | — | Wrist sensors lag behind fast changes and can lock onto your cadence |
| Cycling, rowing, elliptical, weights, boxing, tennis | Chest strap | — | Gripping, arm levers and irregular movement disturb the wrist signal (1, 8) |
| Resting heart rate trend | Smartwatch or ring (overnight) | Phone camera each morning | Consistency matters more than precision |
| Morning HRV reading | Chest strap or phone camera | Smartwatch that takes a deliberate reading | Both agreed with ECG at rest (3) |
| HRV during sleep | Smartwatch or ring | — | You can't sleep with a finger on a phone, and few people sleep in a strap |
| A quick "is my pulse normal right now?" | Your fingers or a phone camera | Any device | A manual pulse also lets you feel the rhythm |
| Checking for an irregular rhythm | A doctor's ECG | A watch with an FDA-cleared ECG feature, then a doctor | Optical heart rate isn't designed to diagnose rhythm problems |
A simple rule: if the number changes fast or you move your arms a lot, use a strap; if you're still, use whatever you'll actually use every day.
How to test your own watch against a chest strap
Studies average many people. Your wrist, your band and your sport are what count for you. You can run a simple check in one or two workouts.
Wear both at once. Put the chest strap on, wetting the electrode pads first, and pair it with a phone app or a second device so you can record it separately from your watch.
Tighten the watch. Wear it snug, a finger's width above the wrist bone, as Apple recommends for workouts (8).
Start both recordings at the same moment.
Do your normal session, including the hardest part, such as intervals or hills.
Note three moments: a steady section, the peak of a hard effort and one minute after you stop.
Compare. If the watch is within a few beats in the steady section but 15 or more beats off at the peak or in recovery, your watch is fine for trends and easy cardio, but use the strap when zones matter.
Repeat for each sport. A watch can be accurate for your runs and unreliable for your spin class.
If both devices disagree wildly even at rest, check the strap first: a dry or worn electrode pad and a low battery are common causes of strange readings.
When a watch is good enough — and when it isn't
A watch is good enough when you want your daily resting heart rate, your sleep heart rate, a rough picture of how hard a walk or easy jog was, or long-term trends. For most people who want to know whether they're recovering, sleeping well or getting fitter, the convenience of something you wear all day outweighs a few beats of error during a workout.
A watch isn't enough when exact numbers change your decisions:
You train in narrow heart rate zones, for example for a marathon or a polarized training plan.
You do intervals where the difference between 165 and 180 matters.
Your sport involves gripping, punching, rowing or swinging.
A doctor has given you a heart rate limit for exercise. The Cleveland Clinic researchers specifically said that people who need precise readings, including those with doctor-set limits, shouldn't rely on wrist monitors (1).
Many watches let you pair a chest strap so the watch shows the strap's numbers during workouts and uses its own sensor the rest of the day. Apple, for example, suggests pairing a Bluetooth chest strap if readings during exercise are inconsistent (8). That gives you the best of both.
The phone camera: a careful resting spot check
A phone camera is the cheapest way to get a heart rate and HRV reading: no device to buy, nothing to charge. It's also the most limited in what it can do.
What it's good for. A short, deliberate reading while you sit or lie still, for example each morning after waking. In the meta-analysis, camera apps agreed well with reference devices in adults at rest (2). In the HRV study, phone camera readings and a chest strap gave practically the same rMSSD as an ECG (3).
What it can't do. It can't track you during exercise, overnight or continuously. It isn't validated in children (2), and camera-based measurement is unreliable in people with arrhythmias (5). It isn't an ECG.
How to get a clean reading (5, 10):
Sit or lie down and rest for 10–15 minutes if you've been moving.
Cover both the camera lens and the flash with one fingertip, without pressing hard.
Keep your hand and finger completely still and don't talk.
Breathe normally; don't try to slow your breathing on purpose.
Use the same finger, position and time of day each time.
If your hands are cold, warm them first.
Not every phone suits this. On some models, the flash sits too far from the camera; Welltory's help center lists Google Pixel phones among them, and suggests a compatible heart rate monitor or Samsung watch instead (5).
We go deeper into camera accuracy in how accurate is HRV from a phone camera and into the no-gadget approach in how to measure HRV without a wearable.
HRV is a different job from heart rate
Heart rate counts beats per minute. Heart rate variability (HRV) measures the tiny differences in time between beats, in milliseconds. That makes HRV far more sensitive to noise: one missed or extra beat can shift the result noticeably, while it barely changes an average heart rate.
That's why the rules for HRV are stricter:
Measure at rest. HRV readings taken while you move aren't comparable with resting ones.
Measure the same way every time. Same time of day, same position, same device. Switching from a strap to a watch midway through a month makes your trend harder to read.
Look at the trend, not one day. A single low reading after a bad night means little; a week-long slide means more. See why HRV fluctuates day to day.
Don't compare numbers across devices. Watches, rings and apps calculate HRV over different windows and at different times (some overnight, some on demand). A ring's overnight average and a morning chest strap reading are different measurements, even if both say "HRV".
For a deeper look at straps specifically, our Polar H10 guide covers fit, battery and a morning protocol.
Common mistakes that make any device look bad
Many "my device is inaccurate" moments are fixable.
With a chest strap:
Dry electrodes. Straps need moisture to conduct. Wet the pads before you start, especially in cold, dry weather.
A strap that's too loose or sits too low on the ribs.
A worn-out strap or low battery. Erratic spikes early in a workout that settle once you sweat are a classic sign of dry contact or an aging strap.
Static from synthetic clothing in dry air can cause brief spikes.
With a watch or ring:
Wearing it loose or on the wrist bone. Move it slightly up the arm and tighten it for workouts (8).
Cold weather. Less blood flows through cold skin, and the sensor may struggle to get a reading at all (8).
Tattoos. Some ink, patterns and saturation can block the light the sensor relies on (8).
Gripping and flexing. Handlebars, kettlebells and push-ups change blood flow in the wrist.
Cadence lock. During running, a sensor can sometimes latch onto your step rhythm instead of your pulse. The Duke study found walking raised error in most devices (6).
With a phone camera:
Pressing too hard, which squeezes blood out of the fingertip.
Moving or talking during the reading.
Measuring right after climbing stairs or rushing around.
Covering only the lens and not the flash.
Who should buy a chest strap
A strap isn't for everyone. Consider one if:
You train with heart rate zones several times a week.
You do intervals, cycling, rowing, strength training or racquet sports and want real numbers.
You want the cleanest possible morning HRV reading.
A clinician has asked you to keep exercise below a certain heart rate.
You probably don't need one if you mainly want sleep, resting heart rate and recovery trends, walk or jog at an easy pace, or already have a watch you wear every day. In that case, the convenience of continuous data is worth more than precision in short bursts.
If you do buy one, check that it transmits over Bluetooth (not only ANT+) if you want to use it with phone apps, and check compatibility with the app you plan to use. Not every strap passes beat-to-beat timing data, which HRV apps need (11).
Living with a heart rate tracker: the questions people ask next
Why does my Apple Watch heart rate jump during workouts?
Optical sensors can lose the pulse briefly during fast changes or arm-heavy movement, then catch up. Tighten the band, move it a little higher on your arm, and pair a chest strap if the jumps happen in a sport where you need accurate zones (8).
Are chest strap heart rate monitors accurate?
In studies, they are the consumer sensor type that comes closest to an ECG. In the Cleveland Clinic study, a chest strap agreed with an ECG at 0.996 (1), and the Polar H10 kept 99.4% signal quality during high-intensity exercise (7). The main failure points are dry electrodes and worn straps.
Does skin tone affect smartwatch heart rate?
In the Duke study, which included equal numbers of people across all six skin tone types, skin tone was not a significant source of heart rate error; type of activity mattered much more (6). Tattoos are a separate issue and can block the sensor (8).
My watch and chest strap disagree. Which one do I believe?
During exercise, believe the chest strap. At rest, they should be within a few beats. If they disagree at rest, check the strap's moisture and battery, and the watch's fit.
Can I use a phone camera instead of a watch?
For a resting spot check, yes. For anything during exercise or sleep, no. Many people use a camera reading each morning to track their trend without wearing anything.
Before you blame yourself
When a watch tells you your heart hit 190 on an easy ride, it's tempting to think you're unfit, or that something is wrong with your heart. Before you blame yourself, look at your data.
The feeling: "My heart rate is way too high for this effort."
What you can check: Does the spike happen only in one sport, or only in the first ten minutes, or only on cold days? Does a chest strap show the same thing?
One lever for one to two weeks: Wear a chest strap alongside the watch in that sport, or tighten and move the watch higher. If the strap shows normal numbers, the issue was the sensor, not you.
The opposite also happens. A watch that reads low during hard intervals can make you push harder than you should. Knowing which tool to trust for which job protects you from both mistakes.
When to see a doctor
See a doctor if:
You feel your heart racing, pounding or skipping at rest, and it keeps happening.
Your resting heart rate stays unusually high or low for you for days without a clear reason such as illness, alcohol or poor sleep.
Your heart rate rises much more than usual during exercise you're used to, or takes far longer to come down.
A watch notification about an irregular rhythm or high or low heart rate repeats.
You feel dizzy, faint or unusually short of breath during exercise.
Call 911 if you have chest discomfort, discomfort in the arms, back, neck, jaw or stomach, shortness of breath, a cold sweat, nausea, or lightheadedness — even if you're not sure it's a heart attack (9).
How to bring this up with your doctor
What to ask for, by name. "Could I have an ECG?" If symptoms come and go: "Would a wearable heart monitor for a few days or weeks help catch it?" If symptoms happen during exercise: "Would an exercise stress test make sense?"
What to bring. Screenshots of the readings that worried you, with the date, time and what you were doing; whether you were wearing a chest strap or a watch; and a list of medicines, supplements, caffeine and alcohol. Note how you felt at that moment.
What to say, out loud. "My watch showed my heart rate at [number] during [activity], and I felt [symptom]. A chest strap showed [number] at the same time." Mentioning both devices helps your doctor judge whether the problem is the sensor or your heart.
If you're told it's just the gadget. Ask: "Even if the device is off, I felt [symptom]. Can we check my rhythm with an ECG so we know for sure?"
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How Welltory helps — and what it cannot do
The limits first. Welltory is a general wellness product, not a medical device. It doesn't diagnose heart conditions, and its readings don't replace an ECG or a doctor's assessment. A camera reading is a spot reading, not continuous monitoring.
What it can do is help you take consistent resting readings with the tool you already have, and see how your days affect them.
Measure with your phone camera or a chest strap. You can take a spot reading of heart rate and HRV with your phone's camera and flash (camera measurements). If you own a compatible chest strap, such as the Polar H10, Garmin HRM-Pro or Wahoo TICKR, you can measure with it instead; the strap needs to send beat-to-beat data over Bluetooth 4.0 (compatibility list). Apple Watch and Samsung watches can be used for both HRV and activity; other watches and rings mainly provide activity data. On the free plan, you get one camera or heart rate monitor measurement per day; Premium, a paid subscription that renews automatically until you cancel, removes that limit (camera measurements).
Measure the same way each time. Welltory's guidance is to measure in the morning after waking, before exercise, food or a shower, in the same position each time (accurate measurements).
Tag what happened. When Welltory detects a stress or recovery episode and asks "What happened?", you can describe it in your own words — "hard intervals", "slept badly", "two coffees" — and the app turns it into tags you can correct. Over time, My Patterns shows which tags come up around your stress and recovery episodes. Some sections, such as month-to-month comparison and full tag details, need a paid plan (My Patterns).
A record for your doctor's appointment. From the web app, you can export your data as CSV: Dashboard → choose a chart → pick the time range → Export (export guide).
How we made it
We based this article on two Cleveland Clinic studies of wrist-worn heart rate monitors, a Duke University study of sources of error in optical sensors, a validation study of the Polar H10 against a medical ECG Holter, a meta-analysis of smartphone camera heart rate apps, a study comparing phone camera HRV with a chest strap and ECG, Apple's guidance on heart rate accuracy, and the American Heart Association's warning signs of a heart attack. Search questions came from what people in the United States most often ask about chest straps, watches and heart rate accuracy. For the product section, we checked every function against the Welltory Help Center on November 6, 2026.
Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.
Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.
Written by Tatsiana Yashyna.
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This article is for educational purposes only and is not medical advice. None of these consumer devices diagnose heart conditions, and an optical reading is not an ECG. Chest pain or pressure, fainting, severe shortness of breath, or a fast or irregular heartbeat with lightheadedness need emergency care: call 911. This guide is published by Welltory, which makes one of the apps mentioned. Welltory is a general wellness product: it helps you track and understand your data and does not diagnose or treat any condition. Sources were retrieved on November 6, 2026.
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Written by Jane Smorodnikova
The founder and CEO of Welltory. A recognized tech leader with two Master's degrees and experience at MIT, she has scaled Welltory to over 17 million users.
Written by Tatsiana Yashyna
Deputy COO at Welltory. With a background in medicine and years of working with health data, she translates research and real physiological signals — sleep, stress, heart rate, and hormones — into clear, evidence-based explanations that help people understand what their bodies are telling them.
References
- American College of Cardiology — Wrist-worn heart rate monitors less accurate than standard chest strap (press release on Gillinov S, et al., presented at ACC.17; published as "Variable accuracy of wearable heart rate monitors during aerobic exercise", Medicine & Science in Sports & Exercise 2017). https://www.acc.org/About-ACC/Press-Releases/2017/03/08/14/02/Wrist-worn-Heart-Rate-Monitors-Less-Accurate-Than-Standard-Chest-Strap
- De Ridder B, Van Rompaey B, Kampen JK, Haine S, Dilles T. Smartphone apps using photoplethysmography for heart rate monitoring: meta-analysis. JMIR Cardio 2018;2(1):e4. https://cardio.jmir.org/2018/1/e4
- Plews DJ, et al. Comparison of heart-rate-variability recording with smartphone photoplethysmography, Polar H7 chest strap, and electrocardiography. International Journal of Sports Physiology and Performance 2017;12(10):1324–1328. https://doi.org/10.1123/ijspp.2016-0668
- Wang R, et al. Accuracy of wrist-worn heart rate monitors. JAMA Cardiology (research letter, 2016). https://doi.org/10.1001/jamacardio.2016.3340
- Welltory Help Center — Taking measurements with your smartphone camera. https://help.welltory.com/en/articles/4412277-taking-measurements-with-your-smartphone-camera
- Bent B, Goldstein BA, Kibbe WA, Dunn JP. Investigating sources of inaccuracy in wearable optical heart rate sensors. npj Digital Medicine 2020;3:18. https://link.springer.com/article/10.1038/s41746-020-0226-6
- Gilgen-Ammann R, Schweizer T, Wyss T. RR interval signal quality of a heart rate monitor and an ECG Holter at rest and during exercise. European Journal of Applied Physiology 2019;119:1525–1532. https://link.springer.com/10.1007/s00421-019-04142-5
- Apple Support — Get the most accurate measurements using your Apple Watch (published March 10, 2026). https://support.apple.com/105002
- American Heart Association — Warning signs of a heart attack. https://www.heart.org/en/health-topics/heart-attack/warning-signs-of-a-heart-attack
- Welltory Help Center — How to take accurate heart rate variability measurements (August 3, 2026). https://help.welltory.com/en/articles/3361520-how-to-take-accurate-heart-rate-variability-measurements
- Welltory Help Center — Compatible devices. https://help.welltory.com/en/articles/4998580
- Welltory Help Center — My Patterns (iOS) (updated September 3, 2026). https://help.welltory.com/en/articles/16012012-my-patterns-ios
- Welltory Help Center — How to export data from Welltory. https://help.welltory.com/en/articles/3368391-how-to-export-data-from-welltory