Apple Watch Series 12: The Best Heart Rate Accuracy of Any Wearable?

Greg Marshall

10 minutes ago

4 min read

A look at what the data tells us about Apple's superlative claim.

Image of watches tested in the study discussed

Apple Inc. made headlines last week with the announcement several new products that will be forthcoming, including a touchscreen flip-phone, the iPhone 18, and perhaps most interestingly, the Apple Watch Series 12. Among the bold claims backing the new Apple Watch, Apple notes that the Series 12 offers “the most accurate heart rate sensing in a wearable.” They even included a link to a new company white paper detailing a study they did to substantiate their claim:

“Apple Watch Series 12 is re-architected to provide continuous high-fidelity health monitoring using the Health Sensing System, featuring new optical and electrical heart sensors. Using this system and the S11 chip, Apple Watch Series 12 offers the most accurate heart rate sensing in a wearable. A scientifically rigorous study on a diverse population of over 1,000 participants compared the new heart rate sensing accuracy on Apple Watch against leading wearables, resulting in Apple Watch showing the highest accuracy across devices.”

For athletes and recreational exercisers, heart rate data accuracy matters a lot. A few beats per minute can be the difference between sustainable exercise that promotes the fitness we’re seeking, and over- or under-cooking ourselves.

I read Apple’s new study, and there are a few details to keep in mind when we evaluate the truth of Apple's superlative claim on heart rate accuracy.

Apple conducted the research themselves.

The paper comes directly from Apple internal testing, rather than a third party tester. It’s clear that Apple wants to showcase a good result for their new product, so the source of the new study itself suggests we should read closely, because there’s a risk that whether knowing or unknowingly, they might introduce bias that could affect their results.

Their subject pool and testing groups sound legit

In total, the study quotes that 1,254 study participants contributed data used in their analysis. They had good sample size in each category of testing, and they made sure to test heart rate accuracy in a large variety of conditions ranging from weight lifting, to HIIT training, to “resting.” All in all, the activities they tested represent the activities people will do in real life with an Apple Watch. They determined the accuracy of each product they tested (Google watch, Garmin watch, Oura ring, Whoop strap, Samsung watch, and a Huawei watch) by comparing them to the results of a Polar H10 chest strap heart rate monitor that was worn by each participant at the same time as the watch.

That last sentence was important. They compared the products to the Polar chest strap.

Apple Watch’s new heart rate is better than every device shown in the study.

Of the seven devices assessed in the study (again, those are Apple Watch, plus Google watch, Garmin watch, Oura ring, Whoop strap, Samsung watch, and a Huawei watch), the Apple watch showed the least error compared to the Polar chest strap of any device tested. Across 159 comparisons with the other devices, the Series 12 performed better in 139.

The bait-and-switch.

The study design itself sounds pretty solid, but they way in which Apple chose to report their results is where we should raise an eyebrow. You can see one example of how Apple displayed the results below:

In every figure used to display their results, Apple shows the accuracy of the competitor relative to Apple’s accuracy. The Series 12’s performance can be seen as the vertical black line at 0.0. The farther the black dot is from the 0.0 line, the worse that devices accuracy was. For example, Oura Ring 5 is farthest to the left, showing the worst accuracy in this (and every other) test.

The problem is Apple Watch was not ground truth. It was not 100% accurate. The Polar H10 chest strap was their ground truth. By displaying all device errors relative to the error in the Series 12, Apple never actually tells us how closely the Apple Watch data agreed with the chest strap. They just tell us that it’s better than the other devices they tested.

The bigger problem is what Apple completely omitted: how their device compares to chest straps or armbands. Every device Apple chose to compare their watch’s accuracy to was at the wrist, or on the index finger for Oura. Previous research has established clearly that chest straps and armbands consistently outperform the accuracy of wrist-based heart monitors. The Apple study dodges that comparison entirely.

The finer points

One last note about the type of error testing (mean absolute error and root mean squared error) Apple used is that it compares each heart rate data point between the watch and the chest strap in any given instant. If the chest strap says 150 beats per minute (BPM) and the watch says 153 BPM, the error is 3 BPM. The error is also 3 BPM if, in the next second, the chest strap says 150 BPM and the watch reads 147 BPM. In other words, we can only trust the watch to within 6 beats per minute, because we don’t know when we’re out on a run if the watch value is 3 BPM high or 3BPM low compared to our real heart rate. When the watch says your heart rate is 147 BPM, it could actually be as low as 144 or as high as 150.

We could go further with this, but things get messy in a hurry. Minimizing error is important, but it’s also important to be directionally consistent in that error (always reporting high or always reporting low, rather than fluctuating between high and low readings).

Bottom Line

Apple has shown somewhat convincingly that they are better at wrist-based heart rate sensing that the six products they compared their product to. It’s almost certainly still worse than heart rate data from a chest strap or an armband, but we don’t get to know since they don’t show us how their device compares to the chest strap they used in testing. They might have also, perhaps unwittingly, convinced us not to buy an Oura ring.