The real world wrist-based heart rate monitor test: Are they accurate enough?

Fitbit, Mio and Basis versus the trusty chest strap
The ultimate wrist based HRM test

Two runners. Three wearables with wrist-based heart rate monitoring. Twenty four graphs. Wareable reports from the front line of running wearables and fitness trackers to investigate whether heart rate tracking watches are accurate enough to be trusted.

It's not surprising that watches with built-in continuous heart rate monitoring have set pulses racing this year. No more stinky, sweaty chest straps wrapped around your torso.

Instead, LEDs on the bottom of the watch peer through your skin and monitor the flow of blood. All very clever and convenient. Over time, you'll also get the added motivation of seeing your resting heart rate decline as your fitness improves.

But are these new heart rate monitors accurate? If not, it's little use for training.

Why we need accurate sensors

Apple Watch specs, release date, price and hardware

Heart rate training relies on exercising in different heart rate zones - each of which is a percentage of your maximum heart rate and stimulates different metabolic pathways and has different effects on the enzymes in the muscles.

The aerobic zone, for example, is generally recommended for overall cardiovascular fitness and is considered to be 70-80% of your maximum. You can estimate your maximum heart rate with a formula based on your age - though, be warned, their accuracy varies. For better accuracy you can take a professional test, which involves a particularly unpleasant workout.

Once you know your zones you can schedule workouts to make the greatest fitness gains without overworking your body, that is if you are measuring the zones correctly.

Chest strap heart monitors have been in use for a while. They detect the electrical activity that is transmitted through the heart muscle to make it contract and are deemed accurate enough, by fitness specialists such as Garmin, Polar and Suunto, to use effectively in training with heart rate zones.

The accuracy of wrist-based heart rate monitoring is still up for debate.

How manufacturers test HRM wearables

While wearable tech companies, including HRM darling Mio, cite studies that demonstrate the precision of wrist-based HR monitoring, we found these studies, often tested the sensors alone, not completed watches. These sensors were then secured under a sweatband, protected from light and movement. That's important, as we'll see shortly.

Often these studies were conducted at running speeds that are slower than most serious runners would be doing in the real world.

Worse, when it came to reporting findings, the studies took an average of walking, jogging and running results. For runners only the third reading is relevant and, from what we saw, it's often the least accurate.

The real world Wareable test

We wanted to know how good the latest generation of sport watches with built-in HR monitors are. We test heart rate tracking in individual Wareable reviews but wanted to focus in on this metric with a dedicated investigation. How accurate are they in real world use? We devised a test to find out.We took two runners, and had each of them run while wearing a wrist-based heart rate monitor and a chest-based one that they had verified as accurate after a period of regular use.

We used two runners because we wanted to get at least some sense of whether different heart profiles would be read differently. Runner one, your author, runs 15-20 miles per week and, while he goes slowly, his heart doesn't. On an economy run, an average HR of 150 - in the upper end of the aerobic zone - is not unusual. He compared the three wrist-based monitors with a Suunto Ambit3 Run and chest strap.

Runner two, Paul Radford, runs ultramarathons and has five wins and four course records to his name. He hasn't finished outside the top 10 since 2011. He runs about 80 miles a week - averaging a bpm count about 114 - and used a Polar V800 with chest strap as his comparison device.

Studying our results would be Sean Radford, Paul's brother, who is a doctor, runner and long-time coach. Sean is the founder of TrainAsOne, currently in beta, which delivers customised training programmes for runners based on data collected from their wearables. He has plenty of experience in assessing training methods, wading through medical reports and making sense of medical assessments of fitness gadgets.

Just to show our working, we've published the graphs we were able to produce from the heart rate data. They show the recorded heart rates, the differences between the wrist based devices and the chest strap and also the overall discrepancy.

A proper scientific survey studying these heart rate monitors would include many runners, strapped to medical-grade equipment and running - not walking or jogging - on treadmills. Our test doesn't provide this but it does at least replicate the experience of real runners. If you buy one of these devices and put it on for a run, how reliable can you expect the data to be?

Adidas miCoach Smart Run

First up was the Adidas miCoach Smart Run, a chunky device with a broad strap that, in white at least, is fairly ugly and bulky. It has a clear display, though the the user interface could be simpler.

Here's why it matters. The Smart Run, and Adidas' latest wearable the Fit Smart both use Mio's continuous optical heart rate monitoring sensor which Mio CEO Liz Dickinson says is more accurate than the tech in rival trackers such as Fitbit devices. It's also the tech Garmin has chosen for its first ever running watch with wrist-based heart rate monitoring tech, the Forerunner 225. High praise indeed.

For your author, averaging 150 bpm, the Smart Run did a good job. It was accurate 90% of the time and was never out by more than five beats per minute. That's a level of accuracy which could definitely replace your chest strap.

For Paul, who averages 114 bpm on his runs, the Smart Run was slightly less reliable. It was in the correct zone 86% of the time and very occasionally - about 1%t of the time - it was out by two HR zones. That's probably on the cusp of viability as a running device. A casual or mid-level runner might accept Mio's 86-90% reliability as a trade-off for ditching the chest strap. A more serious runner, or someone who demands maximum accuracy, might not.

Basis Peak

Basis Peak

The next device was the Basis Peak, which will track runs as well as monitoring all-day activity and sleep. While it's true that looks are subjective, we have to say this watch is no looker, either. It feels a little cheap. Also, perhaps because of its role as an all-day activity tracker, the Peak doesn't offer the kind of detailed information a runner wants.

Basis says it combines an optical sensor with special algorithms to continuously measure heart rate 24/7. It has done internal testing and is in the process of getting third party validation.

Is the Peak's HRM accurate? Not brilliantly. Your correspondent's readings were in the correct heart rate zone just 70% of the time and for a small portion of the run - two per cent - they were two heart rate zones out. The readings varied more, too, and were often 10 beats per minute out.

The Basis worked a little better for Paul. It took some time to pick up his run and was under-reading by about 18 beats at first; you don't start a run on the Basis, it notices the activity by itself. Once it did pick up the run, however, it stayed within five beats and was in the right zone 75% of the time. Even so, neither one of us would be happy to replace chest strap data with that.

Fitbit Surge

Finally, we turned to the Fitbit Surge.

This was the best looking of the three devices, though its small screen doesn't fit much information - not ideal when you are on the move. It was simple to set up but, once again, accuracy was suspect.

As for the heart rate tracking, the Surge uses Fitbit's PurePulse tech which can also be found in Wareable's current best overall fitness tracker the Fitbit Charge HR. The Charge HR was our first choice for this in-depth test but as we weren't able to export the full data (just daily totals), we took the Surge out running instead.

So how did it do? The Surge seemed to take about five minutes to properly pick up the heart rate for your correspondent. Once it did, it was in the right heart rate zone 77% of the time.

Paul's test with the Fitbit saw it perform marginally better. It took eight minutes to settle down and start reading accurately but once it did, it was in the right heart rate zone 82% of the time.

Back to the chest strap?

As we've said, this experiment was not a scientific test but if it was, it would be one of those that concludes more research is needed. These three devices do not appear to match the accuracy of a chest strap, with the best performance we saw being Mio's HRM tech at 10% out.

Why aren't these devices as accurate as a chest strap? One clue is that they seem to be more accurate when not running and, in the studies we've seen, they are more accurate for slower runners.

One educated guess is that the movement of the arm is to blame. Generally the faster you run, the faster your arms move and this may either cause the device to shift on the wrist, hampering accuracy, or perhaps stop the sensor from getting a clear view of blood flow.

It's also possible that small discrepancies in the fit are magnified when you are in motion. Though we followed the manufacturer's recommendations in putting the devices on, perhaps a slightly too tight or too lose watch struggles when you are running.

More worrying is why the two of us saw different results with the same device. Perhaps the difference in our heart profiles accounts for that - two of the three devices, the Fitbit and Basis, were more accurate for Paul's lower bpm, whereas the Adidas was vice versa. Differences in wrist shape could also be partly to blame and as we said, particularly with Mio's tech fit is important to get the correct readings.

As for Wareable's verdict, if you don't mind sacrificing 10-15% accuracy versus a chest strap, the Adidas miCoach Smart Run is clearly the best of the bunch, and the just-tested Garmin Forerunner 225 which uses the same sensor is worth considering.

If you are in the market for a running watch and heart rate monitoring is of secondary importance then these devices are fine; you will get a reasonable estimate of your average heart rate for a run.

However, if you are doing training based on heart rate zones or you want the most accurate data possible then we cannot recommend them. Our doctor/runner/coach Sean Radford's verdict is that having already been skeptical about these wrist-based heart rate monitoring devices, he still isn't convinced that they are ready for serious runners.

In other words, you'll be wearing that stinky, stretchy chest strap close to your heart for a while yet.


  • thesglife says:

    Thank you for the review!

    • hjkhjhl says:

      This is a pure shit site. Just shilling for manufacturers. Any serious test of accuracy would have included the Rhythm+. When you look at the recommended list there's a slew of very pricey fitness trackers that everyone and their mother knows are not accurate. Yet these clowns have no problem recommending that you spend 250+ pounds or more on them. 

      This is truly a fluff site with no real content and if this "accuracy" article was to bring some form of credibility to this site, it utterly failed. It was a weak attempt that may have only fooled the sorts of imbeciles that value good site design over actual content. No doubt this site is well done and very slick looking.

  • dalej1969 says:

    Great test. Would love to see it done with mio alpha 2, apple watch, schosche rhythm plus


  • stevespeirs says:

    Agree with dalej1969 - would like to see data for the Scosche RHYTHM+

  • WorkOutCancer says:

    Thanks!  Would love to see ANYTHING on the Apple Watch.  I just started testing the Scosche Rhythm, first run it loosened without me noticing it (I wore it on upper arm) and HR looked erratic in Garmin Connect graph.  I'm looking for a reliable iphone app that I can collect HR with instead of having to wear the Garmin Forerunner along with the Scosche Rhythm.

  • Carolsuej says:

    I too just want a wristwatch HRM to let me know when I go into AFib. Is there a wrist monitor that will provide this information. 

    Thank you,


  • Xames says:

    love this but wish the Apple Watch was included, you used an Apple Watch picture, so why no actual watch?

    • j.stables says:

      Problems with actually exporting the Apple Watch data. Working on an update...

  • Harry says:

    "HRM capable of detecting my going into AFIB" The new Microsoft smart watch might be usable here. It supposedly measures the time between contractions as well as HR - which might/could be useful in deciding if AFIB is happening with or without an app running on a smart phone. I have Paroxysmal, or intermittent AFIB - so that's my main interest in a smart watch. I am going to check it out.

    • SBL says:

      Hi Harry! did you test teh Microsoft smart watch? Did it work? Thank you, SBL

  • Harry says:

    "HRM capable of detecting my going into AFIB" The new Microsoft smart watch might be usable here. It supposedly measures the time between contractions as well as HR - which might/could be useful in deciding if AFIB is happening with or without an app running on a smart phone. I have Paroxysmal, or intermittent AFIB - so that's my main interest in a smart watch. I am going to check it out.

    • SBL says:

      Hi Harry. Did you try the Microsoft smart watch? Did it work??? Thank you for your reply, SBL

  • Vincy says:

    No offense but your writing style is hard to understand. Please read your article back and you'll see what I mean. Thanks for the information though. 

  • PMall says:

    Thank you for the studies.  I agree with them.  I have been a runner and cyclist for 25+ years and have used a Polar Chest Strap HRM for years.  I have always been pleased with the accuracy of the Polar HRM, but I thought it might be nice to get rid of the chest strap.  So I did what I thought was thorough research and bought a Fitbit Surge.  I liked the GPS feature, the sync to my phone and laptop, etc.  I figured it would be worth $250.  But I have been very disappointed with the inaccuracy of Fitbit's HRM.  I frequently do interval/zone training, and I can usually guess my BPM within 5-10 beats.  I have done several side-by-side comparisons by wearing both the Fitbit Surge and my Polar chest strap during the same workout and cannot believe the discrepancies.   Recently, well into a run, I guessed my BPM to be ~130 BPM.  I checked and the Polar was at 132 (which I believed), and the Fitbit was at 176!  No way!  That's significantly beyond my anabolic threshold, and I wasn't feeling the least bit stressed at the time.  Ridiculous! 

    I have communicated several times with Fitbit about this issue.  They had me verify all the standard questions (not too loose, not too tight, correct spot on the wrist, etc).  They had me reboot the device twice.  They concluded that my Fitbit was defective and sent me a new Surge.  Guess what?  Exact same thing!  ........Looks like I'll be wearing the chest strap for a long time!

    • snafunaafi says:

      If like me you have Proximal Atrial Fibrilation, you know how hard it is to find what causes the crazy electrical signals that high jack the heart.  Gym wise I stop when these signals send my heart over 160BPM.  Never use a Fitbit, its a toy.  Readings from my Polar and gym equipment gave a HR of 210, Fitbit was happy to give 95 BPM.

      If anyone finds a watch that works let me know.

      • rlangton76 says:

        The fenix HR 3 from Garmin is amazing. I did 2 runs on back to back days, one with the HR chest strap, and one just relying on the watch HR monitor. 

        Day 1 - With HR strap - Avg HR 144, Avg pace 8:59, 3.01m
        Day 2 - W/O HR strap - Avg HR 145, Avg pace 9:06, 3.68m

        The charts are nearly identical, with no spikes or drop offs occurring.

  • Gurulee says:

    Any tests and improvements with accuracy using the Garmin forerunner 235? 

  • BullHorn says:

    its been a while since this was posted and many new models were released. Are you planning to make a new test?

  • Lucious says:

    thanks for the test!!! Would love to see the same test conducted for scosche rhythm+


  • paull says:

    Thanks for Nice reviews. The wrist HRM would be accurately medical usage compare to ECG or Chest Strap ones. Simply due to green light absorption under skin would be subject to water, air, sweat, thickness of skin, blood flow accretion rate....etc affects. 

  • Davide says:

    I gave up using the Charge HR when training for a 50k hard course mountainbike race it showed up complete discrepancy of the HR reading, and that was not compared with another monitor but with me taking my radial pulse, to the extent that after a very steep section my pulse was 175 and the Charge was showing 112bpm. and its not because of movement, because I was still. I attributed to the sweat on the skin, as possibly altering the conductivity.

    I tried the Fitbit in the shower then and again was reading at 112bpm while my pulse was in the 70's.

    bloody useless gadgets

  • astroyam says:

    I have been using the Garmin Forerunner 225, with very little success. I am a runner, and during the first 10 minutes while I am warming up very easy, the device may read 190-200 bpm and higher, while my heart rate is about 120. My max heart rate (measured at the Boulder Center for Sports Medicine) is 188 bpm. Randomly during runs the device may read 50-60 beats high. It's basically not of use in an obvious way about 10-30% of the time, which of course makes it impossible to say if it's correct the rest of the time, since I'm not wearing 2 watches to benchmark the first... I don't consider wrist HRM to be a realistic technology at this time. I'm sure it will get better though.

  • Nenadn says:

    Is there any study on possible negative effects wearing chest strap as it constantly sends electromagnetic signals to the watch?

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