Rule of Twelfths
The Rule of Twelfths is a quick way to estimate the height of tide between high water and low water when a full tidal curve is not needed. The range is divided into twelve parts. In the first hour the tide rises or falls about 1/12 of the range, then 2/12, 3/12, 3/12, 2/12 and 1/12 in the six hours between HW and LW. It assumes a regular semi-diurnal tide and a six-hour stand between high and low water.
How this calculator works
Range = High Water − Low Water. The tool adds or subtracts the twelfths for the number of hours after HW or LW. Hours should be between 0 and 6. This is an approximation. Ports with a stand of tide, a double high water or a large diurnal inequality need official tide tables or software.
Who should use it
Useful for officers estimating underkeel clearance or lock times when only HW and LW are known. Always prefer the official tide table for the port when available.