How to Calculate the Height of Tide at a Given Time — Worked Example

Find the range (HW height minus LW height), split it into twelfths, and add up the twelfths that have changed since high or low water: 1, 2, 3, 3, 2, 1 for each hour. Three hours after a 5.3 m high water with a 0.9 m low, the tide has fallen 6 twelfths and the height is 3.1 m.

The question

High water is at 0930 (5.3 m) and low water at 1530 (0.9 m). What is the height of tide at 1230?

What you are given
Item	Value
High water	0930, 5.3 m
Low water	1530, 0.9 m
Time wanted	1230

Tide curve drawn with the Rule of Twelfths. Real curves vary from port to port; use the almanac curve for real passages.

Working, step by step

Range = HW − LW = 5.3 m − 0.9 m = 4.4 m.
One twelfth of the range = 4.4 m ÷ 12 = 0.37 m.
1230 is 3 hours after HW. In those 3 hours the tide falls 1 + 2 + 3 = 6 twelfths.
6 twelfths of 4.4 m = 2.20 m.
Height of tide = 5.3 m − 2.20 m = 3.1 m.

Answer

The height of tide at 1230 is about 3.1 m above chart datum.

The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.

Try it yourself

Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.

Learn the method

More worked tide examples

Frequently asked questions

Does the Rule of Twelfths work at every port?

Only roughly, and only where the tide is regular with about six hours between high and low water. At ports with unusual tides, such as the Solent, use the port's tidal curve from the almanac instead.

Why is the height measured above chart datum?

Tide heights in tide tables and charted depths are both measured from chart datum, roughly the lowest astronomical tide, so you can add them to find the actual depth.