Measure Heart Rate With Phone's Camera Using Google Fit App 

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Install Google Fit app from App Store or Play Store.

Open Google Fit. Scroll down and tap "Get Started" under Check your heart rate.

Tap Next and follow on-screen instructions.

Place Finger on phone's rear camera to start measuring heart rate.

Google Fit will show your heart rate within a few seconds.

The results were quite accurate compared to Apple Watch and Oximeter.

The accuracy may vary across phones. Also, the feature is not yet available for all phones.