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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. Learn what else affects your reading.

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copied from: Monitor your heart rate with Apple Watch - Apple Support

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Many factors can affect the performance of the Apple Watch heart rate sensor. Skin perfusion (or how much blood flows through your skin) is one factor. 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 might be too low for the heart rate sensor to get a reading.

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.

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.

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. Learn how to pair Bluetooth accessories.

Heart rate is 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.

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copied from: Get the most accurate measurements using your Apple Watch - Apple Support