The Rule of Twelfths is an approximation to a sine wave curve. The formula can be used as a rule of thumb for estimating a changing quantity where both the quantity and the steps are easily divisible by 12. It has been typically used for estimating the height of the tide. The rate of flow in a tide increases smoothly to a maximum halfway point between high and low tide, before smoothly decreasing to zero again. The rule is also used to make predictions on the change in day length over the seasons.

Tidal changes are non-linear. This means that in the first hours of a tidal shift the tide might not rise or fall very much, yet as the cycle progresses the rising or falling will accelerate through the mid hours. The Rule of Twelfths applies to the semidiurnal tide - a tide having two high waters and two low waters during a tidal day, which is exactly what happens in most locations. The semidiurnal tide period lasts for a period of 12 hours and 25.2 minutes from low to high tide, and then repeats back to low tide again. The full and new moons also have effects on the tide, as do the first and third quarter moons.