Rule of Twelfths

The Rule of Twelfths explained

The Rule of Twelfths is a handy rule of thumb for estimating tidal heights. You will 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, increases to maximum in the middle two hours and then decreases to nil at High Water.

This image shows the progression of the Tide as assumed by the Rule of Twelths.
Rule of Twelfths Hour by Hour

Worked Example

Diagram showing a worked example of how to use the Rule of Twelths
Rule of Twelfths Explained

The example shown above is within about 20cms of the answer derived from the curve. For an explanation of Tidal Curves Click here.

This is a rough and ready system and should 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 or The Solent where the tidal curves are 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.

For explanations of Tidal Streams Click here.

For explanations on Secondary Ports Click Here

To look at our Cockpit Cards Click Here