When measuring your Heart Rate, the result is incorrect, inconsistent, or doesn't display.
Make sure your watch is comfortably snug on your wrist, with the sensor completely touching your skin. Stay still until the measurement is complete, about 8 seconds. When the result displays, that is your real-time heart rate.
If you the heart rate measurement fails, try again. If it continues to fail, restart your watch by pressing and holding the Home button, then tap Restart.