How do you check your heart rate
With watchOS 3, you can check your heart rate any time using the Heart Rate app. Open the app, then wait for Apple Watch to measure your heart rate.
When does Apple Watch measure your heart rate
When you use the Workout app, Apple Watch measures your heart rate continuously during the workout.
This information, as well as other data it collects, helps Apple Watch estimate how many calories you’ve burned. In addition, Apple Watch measures your heart rate throughout the day when you’re still. You can see these background heart rate measurements in the Health app on your iPhone. Since Apple Watch only takes this background reading when you're still, the time between these measurements will vary. You can control which third-party apps have access to your health data from the Health app in Sources.
How Apple Watch measures your heart rate
The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.
The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate in the background.
For best results, start with a good fit
Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all. But there are things you can do to help Apple Watch get the most consistent and best heart rate readings possible.
If your Apple Watch doesn’t stay in place, or the sensors aren’t reading your heart rate, tighten the band a bit.
Your Apple Watch should be snug but comfortable.
What else affects your reading
Many factors can affect the performance of the Apple Watch heart rate sensor. Skin perfusion is one. A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading.
Motion is another factor that can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
If you’re not able to get a consistent reading because of any of these factors, you can connect your Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps.
When doing an Open Water Swim* in the Workout app, water may prevent a heart rate measurement, but Apple Watch will still track your calories using the built-in accelerometer. When doing a Pool Swim* in the Workout app, water may prevent a heart rate measurement, but Apple Watch will still track your calories, laps, and distance using the built-in accelerometer.
Heart rate is just one of many factors that Apple Watch uses to measure your activity and exercise. Depending on your workout, it selects the most appropriate inputs for that activity. For example, when you’re running indoors, it also uses the accelerometer. When you’re cycling outdoors, it uses the GPS in your Apple Watch Series 2 or in your iPhone for your Apple Watch (1st generation). And even when you’re not in a dedicated workout, it tracks how much you move each day. So Apple Watch can give you the information — and the motivation — to improve your fitness and your health.
* Available on Apple Watch Series 2 only. Apple Watch Series 1 and Apple Watch (1st generation) aren't suitable for swimming. Learn about water resistance on your Apple Watch.