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Apple caught my attention with their claim that the Apple Watch Series 12 and Ultra 4 have the “most accurate heart rate in a wearable.” It’s hard to make a really accurate heart rate sensor, and in my experience, Apple hasn’t had a great track record in that area. So I tested the Series 12 pretty intensively over the past week or so, and now I have results.
I’m picky about accuracy; longtime readers have seen me test watch after watch against my trusty chest strap to find out how good each one really is. That sometimes brings surprises: The Powerbeats Pro 2 are surprisingly bad at heart rate tracking if you have small ears, for example. I also discovered that the Garmin Forerunner 570 is shockingly good at heart rate accuracy, far better than any other device I had tested up to that point. (Garmin’s Forerunner 970 and the newest Fenix and Venu watches have the same sensor.)
What Apple says about their heart rate sensor, and why I doubted it
Is the new Apple Watch really good enough to beat Garmin’s best sensor? Apple believes it is, but I doubted that claim, for a few reasons. The first is simply Apple’s track record. Early models of the Apple watch had better sensors than most wearables at the time, so Apple developed a reputation for good accuracy. For example, back in 2019 or so, the Series 4 watch performed far better for me than the Whoop 3.0. But time marches on, and devices have gotten better. Garmin has an excellent sensor, as I already explained; the Pixel watch is also impressively good. But Apple just didn’t keep up.
You can see it in my comparisons over the years: here’s the Series 11 glitching where a Garmin did just fine, and here’s the Series 10 struggling alongside a Fitbit Charge 6. If I were to rank all the heart rate sensors I’ve tested for their accuracy, the Garmin 570 would be A-tier, the Pixel Watch and most good sports watches would get a B, and the Apple Watch (Series 10 and 11) would be a C. (S-tier would be reserved for chest straps.) Not long ago, I got downvoted to negative infinity for commenting on Reddit that Apple’s heart rate accuracy (up to the Series 11) is “mid at best.” Sorry, but it’s true.
But, OK, perhaps it’s a new era. Apple redesigned its optical heart rate sensor to include larger LEDs and arrange them more closely in a circle. To support its claim that the new sensor is the most accurate on the market, it published a study that was extensive in its breadth (seven recent, competing devices were tested in a variety of activities) but limited in usefulness. Each pairing of a device and workout type was given, essentially, one number. We don’t get to see how each device behaves in use. The data feels cherry-picked, and in any case, it doesn’t give us enough information to say whether one sensor is truly better than another, or specifically what it does better. So I’m going to add my own data points.
How I tested the Apple Watch’s heart rate sensor’s accuracy
I am just one person, so my experiments amount to a very small, informal study. That’s the case for any tech reviewer, no matter their qualifications—so feel free to compare my data to what others have found, but keep these limitations in mind for each of us.
I took the Apple Watch Series 12 and the Garmin Forerunner 570 out for several tests during workouts. (I did not have a feasible way of testing background measurements outside of workouts.) I wore one device on each wrist, positioned according to manufacturer recommendations, and, at the same time, used an electrical chest strap to provide the reference values.
I have relatively light skin, which may improve accuracy, and no wrist tattoos, so both sensors should get a clear signal on either wrist. I have six-inch wrists, on the small side of average, so I wore the 42-millimeter size of each watch (both are available in larger sizes for those who prefer it). A good fit is critical to getting good accuracy, and I feel I achieved that for both devices. I did my best to have both devices equally snug (but not uncomfortably tight) against my wrist.
I tested both devices, together, on treadmill runs, outdoor trail runs, a rowing session, and a few strength sessions. Some of these workouts involved quick changes in intensity, leading to peaks and troughs in the graph. Others were more steady. Gripping objects such as dumbbells or rower handles can interfere with accuracy for some devices, which is why I included those workouts. Ambient light and temperature can affect accuracy as well, so I tested in a variety of weather conditions.
The results
In my first set of tests, which I also shared here, the Apple Watch and the Garmin both performed well, but each had their flaws. The Garmin sometimes lagged a bit at the start of the interval, but quickly caught up. The Apple Watch didn’t have that problem, but sometimes missed the highest values on small spikes in heart rate. I’d call this one a tie.
Apple Watch in red, Garmin in purplish, chest strap for reference in gray.
Credit: Beth Skwarecki
In my other trail runs and treadmill runs, I saw a similar pattern: Both watches would be on track most of the time, but each had their quirks. Garmin was more likely to miss out on the beginning of spikes, but the Apple Watch was more likely to drop data points entirely, especially toward the beginning of a workout.
One day, I did three mini workouts at the gym: a treadmill run, a rowing session, and a strength session. The strength work included a few sets each of overhead barbell press, front squats (with my arms in a clean grip), and dumbbell curls. Here are those graphs, again with the Apple Watch in red, the chest strap in gray, and the Garmin this time in purple:
Apple Watch in red, Garmin in purplish, chest strap for reference in gray.
Credit: Beth Skwarecki
Both devices performed equally on the treadmill run, with their own minor glitches but both getting the job done pretty well. On the rower, the Garmin struggled more, missing almost half of the second interval. The Apple Watch wasn’t perfect, though, overestimating in a few places and not always picking up the lowest value between intervals.
The strength workout was a mess for both devices. The Garmin often read low, but the Apple Watch sometimes read high and reported fewer values in total. This view shows it a bit better (each data point is a dot):
Apple Watch in red, Garmin in purplish, chest strap for reference in gray.
Credit: Beth Skwarecki
So, is Apple’s sensor really the most accurate in a wearable?
Looking at my data, I think the biggest news is that Apple is no longer lagging when it comes to heart rate accuracy. The Series 12 is a lot more accurate in my tests than the Series 10 or 11 ever were. Apple has built a very good heart rate sensor, and they deserve huge credit for that. I will no longer have to tell people Apple's sensor is mid.
But does it have the best heart rate sensor? I’m not so sure I’m convinced. It is holding its own against Garmin’s best sensor, for sure, and that’s impressive. In the rare cases the Garmin struggles, the Apple is usually there with a more-or-less correct value. But I have seen near-perfect graphs from the Garmin on its best days (admittedly, outside of this testing week); I've never seen one quite like that from Apple. The Apple Watch's sensor sometimes has trouble ramping up to correct values at the beginning of a workout; it overestimates on occasion; it doesn’t get all the way down to low values when intervals change quickly; and I’m concerned about the sparseness of the data points. It almost seems like the Apple Watch won’t report a value it’s not sure of, which is arguably better than recording a data point that could be wrong, but leaves odd gaps in some of the charts.
This leads me to over-analyze the claim of “most accurate in a wearable.” If Apple’s sensor fails to report a data point where another device reports an incorrect one, does that make Apple “more accurate” because more of its data points were on the right line? That’s probably a fair claim, but it still doesn’t feel great to award that device the “best” title. And if we truly want to compare the Apple Watch to all wearables, shouldn’t we consider chest straps, which are also worn, and which are objectively more accurate than any optical sensor in a watch?
After all this analysis, I have a hard time awarding the Apple Watch a definitive title of being the “best” at wrist heart rate accuracy. But it’s damn good, and I have to put it up in the A tier with the Garmin. Congratulations, Apple. You managed to tie the best watch sensor out there, and I can’t claim there’s another one that’s better.
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