Rule of Twelths explained



The Rule of Twelths is a rule of thumb for estimating tidal heights.  You only need to know the Time and Heights of High and Low Water.  It assumes a six hour period from Low Water to High Water and that the rise starts slowly, increasing to maximum in the middle two hours and then decreases to nil at High Water.

Rule of Twelths Tidal Calculations



Rule of Twelths Tidal Heights


The Rule of Twelths must always be used with a certain amount of caution, it will work well where the curve is regular, such as the Plymouth Curve shown elsewhere on this site, it will not work at all in places like Poole where the curve is far from regular.

As with other tidal calculations it is never exact as influences such as barometric pressure and the weather can all effect the actual tidal heights and times.

The example shown is within about 20cms of the answer derived from the curve.