Tides are known to be mainly caused by the moon's gravity. Theoretically, peak tidal forces at a given location occur when the moon is at the meridian, but there is usually a delay before high tide that depends largely on the shape of the coastline, and the sea floor, therefore, the lunitidal interval varies from place to place. The lunitidal interval further varies within about +/- 30 minutes according to the lunar phase.
The approximate lunitidal interval can be calculated if the moon-rise, moon-set and high tide times are known for a location. In the northern hemisphere, the moon is at its highest point when it is southernmost in the sky. Lunar data are available from printed tables and online. Tide tables tell the time of the next high water. The difference between these two times is the lunitidal interval. This value can be used to calibrate certain clocks and wristwatches to allow for simple but crude tidal predictions.
 See also
- HWI Datum table for locations in the US
- HWI table for UK
- HWI map for france
- Table of values for Australia
- Values for the Netherlands
- Calculator for android
- Online Calculator