Rule of Twelfths - Example Calculation

Example Calculation

If a tide table gave us the information that tomorrow's low water would be at noon and that the water level at this time would be two metres above chart datum and further, that at the following high tide the water level would be 14 metres. We could work out the height of water at 3:00 p.m. as follows:

• The total increase in water level between low and high tide would be: 14 - 2 = 12 metres.

• In the first hour the water level would rise by 1 twelfth of the total (12 m) or: 1 m

• In the second hour the water level would rise by another 2 twelfths of the total (12 m) or: 2 m

• In the third hour the water level would rise by another 3 twelfths of the total (12 m) or: 3 m

• This gives us the increase in the water level by 3:00 p.m. as 6 metres.

This represents only the increase - the total depth of the water (relative to chart datum) will include the 2 m depth at low tide: 6 m + 2 m = 8 metres.

Obviously the calculation can be simplified by adding twelfths together and reducing the fraction beforehand i.e.

Rise of tide in three hours