Just how accurate is the Apple Watch heart rate monitor? It’s on par with an EKG

Engineer Brad Larson strapped on his Mio Alpha heart rate monitor, one of the most accurate monitors around, to compare its data to the Apple Watch’s results during a run. He posted the graph on Twitter, and the Apple Watch matches the Mio Alpha at nearly every data point. The two line graphs are almost perfectly synced, with only a few variations.

Extracted the raw Apple Watch HealthKit heartrate samples after a run and compared it to an HR monitor I had on: pic.twitter.com/2Ycj49Ctjt

— Brad Larson (@bradlarson) May 7, 2015

Seeing as the Mio Alpha is rated at EKG level standards of accuracy, the fact that the Apple Watch manages to match it so closely is very impressive. Most wearables return results that are very far off from heart rate monitoring chest straps, which are typically considered the most accurate. In our review of the Fitbit Charge HR, we found it to be 20 to 30 beats per minute higher than the chest strap rates during exercise, and between 5 to 15 beats higher during regular walking. That’s also been the case with smartwatches like the LG G Watch R, which took a few weeks to get close to accurate results.

Of course, the Apple Watch’s heart rate monitor has had its own issues, mainly when it’s been worn by people with dark tattoos on their wrists. The watch uses photoplethysmography technology, which uses green LED lights and light‑sensitive photodiodes to detect the amount of blood flowing through your wrist.

Normally, this tech works very well, as evidenced by Larson’s results, but dark tattoos absorb the light, interfering with the reading. There must be at least a few wrist-tattooed Apple Watch owners today who wish they had listened to their mothers.