How to measure heart rate on your smartwatch

Modern smartwatches have a wide array of heart-rate monitoring features. That includes passive all-day monitoring as well as spot checks for specific metrics. Some will even alert you if your heart rate suddenly spikes when it shouldn’t. The most advanced devices also have FDA-cleared electrocardiograms that may help detect atrial fibrillation.

These are impressive features considering that, just a decade ago, smartwatches were basically pedometers. In practice, heart rate monitoring can be a helpful tool for people hoping to understand more about their bodies or measure their fitness progress over time. For instance, some athletes prefer to train in heart rate zones — which is hard to do unless you can check it in real time. There have also been several stories of people who say their smartwatches saved their lives thanks to abnormal heart rate alerts.

That said, it’s important to remember that these aren’t medical devices. At the end of the day, they’re not capable of diagnosing you with any sort of condition. And, even if they were, doctors are still on the fence about how to handle wearable data that consumers collect on their own.

There are nuances to how these features work and plenty of reasons why you might get inaccurate readings. First, we’ll cover how heart rate monitoring works and how to measure your heart rate. Then we’ll get into what EKG-capable smartwatches can and cannot do and how to use the EKG feature with the current crop of smartwatches.

Vjeran Pavic / The Verge

How optical heart rate sensors work

The vast majority of wearables use a photoplethysmography (PPG) sensor — the green LEDs you see when you flip the device over. The LEDs shine light into your skin; the light that’s refracted back is translated by an algorithm into heart rate data. That includes your resting heart rate but also other metrics like VO2 Max and heart rate variability.

While optical heart rate sensors have gotten better over the years, wrist-based PPG sensors still aren’t always accurate. A recent study found that these sensors didn’t work as well on darker skin or people with obesity. The wrist is also not the best place for a PPG sensor. There’s a lot of noise from your arm movements, and a company’s algorithm has to be able to effectively filter that out. That’s why you could wear the same smartwatch on each arm and still get slightly different readings.

Fit is also an important factor. It varies from smartwatch to smartwatch, but here are a few general tips for getting the most accurate data possible.

• Wear the watch about one finger length above your wrist bone
• Make sure there’s good contact between the sensor array and your skin
• Tighten the strap during exercise. You may also want to wear the device higher on the wrist.
• Wipe the sensor array clean whenever it gets dirty