Rule of Twelfths Explained

The Rule of Twelfths is a traditional way of estimating how much the tide will rise or fall during each hour between Low Water and High Water, or between High Water and Low Water. It is a Rule of thumb. The rule assumes there are 4 tidal periods a day (ie one from low water  to high water, a second from high water to low water and so on and ghat each period is approximately 6 hours.

It is based on the idea that, during a typical six-hour tidal period, the water level changes slowly at first, fastest around the middle of the tide, and then slows again as it approaches the next High or Low Water.

The total tidal range is divided into 12 equal parts. The approximate change in height during each hour is then:

A useful way to remember it is:

1 – 2 – 3 – 3 – 2 – 1

For example, if Low Water is 1.0 m, High Water is 7.0 m, and the tidal range is therefore 6 m, one twelfth of the range is:

6 ÷ 12 = 0.5 m

The estimated rise would therefore be approximately 0.5 m in the first hour, 1 m in the second, 1.5 m in the third, 1.5 m in the fourth, 1 m in the fifth and 0.5 m in the final hour.

So three hours after Low Water, approximately half the tidal range would have risen:

1 m + 3 m = 4 m

The Rule of Twelfths is useful for quick passage-planning calculations, estimating depth over a harbour entrance or drying area, and deciding when there may be sufficient water to enter or leave a berth.

However, it is only an approximation. It works best where the tidal curve is reasonably close to a simple sinusoidal shape. Local geography can significantly distort the tide.

Certain places, such as our home, Southampton have unique features which break this rule. Specific details are at Understanding Southampton Tides.

For accurate calculations, especially where under-keel clearance is limited, use the published tidal curve or tidal prediction for the location rather than relying solely on the Rule of Twelfths.