Rule of twelths

Tidal Heights

A useful guide to working out very rough tidal heights is the rule of twelfths. To use this rule take the range of tide in metres between low and high water and divide this number by twelve. It is then assumed that in the first hour before or after high or low water the tide will rise or fall by one twelfth.

Two hours before or after high or low water the tide will rise or fall two twelfths. In the third and fourth hours, before or after high or low water the tide will rise or fall three twelfths. So if HW is 7m at midday and the range before the next low water is 6m, the tide will be 6.5m at 13:00, 5.5m at 14:00, 4m at 15:00, 2.5m at 16:00, 1.5m at 17:00 and 1.0m at 18:00.