This calculator calculates tidal gates using the rule of twelfths to find out tidal gate open and close times.
The rule assumes that the rate of flow of a tide increases smoothly to a maximum halfway between high and low tide before smoothly decreasing to zero again and that the interval between low and high tides is approximately six hours. The rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1
Calculator uses rule of twelfths to find out hour points and when uses linear approximation to find out tidal gate open and close time inside the hour.
Note that the rule is a rough approximation only and should be applied with great caution when used for navigational purposes.
For example, this rule assumes that all tides behave in a regular manner, this is not true of some geographical locations, where there are double high waters or double low waters. Also this rule assumes that the period between high and low tides is six hours but this is an underestimate and can vary anyway.
Officially produced tide tables should be used in preference whenever possible.