The compass in Suunto Core automatically calibrates itself as you use it, but it may need to be calibrated manually in the following circumstances:
To calibrate the compass, enter compass mode. While hold the watch level, and start slowly rotating it. 3-4 full rotations should normally be enough to calibrate the compass. If the watch has been near a magnet/magnetic field, you may need to rotate it a few more times. Keep rotating compass reading appears.
NOTE: The compass deactivates after 60 seconds. This is a power saving feature. Press the Start/Stop button to reactivate the compass.