How to Read a UK Tide Table: A Beginner's Guide

A tide table is one of the least glamorous things you can take to the coast, and one of the most useful. Read it properly and you know whether there will be water under the boat, sand under your feet or a channel you can paddle at two in the afternoon. Read it carelessly and you spend an hour on a harbour wall waiting for the sea to come back.

The numbers are not difficult once you know what each column is for. Here is how a UK tide table works, what the heights are measured from, and how to turn all of it into a plan for the day.

What high and low water mean

A tide table predicts the times and heights of high water and low water at a named place. High water is the peak of the tide, low water the trough. Between a low and the next high the tide is flooding; between a high and the next low it is ebbing. The moment it turns, with almost no movement either way, is slack water.

Most UK coasts get two high waters and two low waters in roughly 24 hours and 50 minutes, which is why tide times drift about 50 minutes later each day. Fish a mark at 09:00 on Saturday and the tide will be doing something similar at about 09:50 on Sunday, and near enough to 10:40 on Monday. That daily drift is not a rounding error. It is the reason a favourite low-water rock pool can be a metre underwater by the following afternoon.

Reading the columns

A typical UK tide table is a list of events. It may have separate columns for high water and low water, or one chronological column with an H or L beside each entry. The exact layout varies, but the important pieces are the same.

• Date and day: the calendar day, often with the moon phase or spring/neap marker nearby.
• Time: the predicted time of high or low water. Check whether the table uses GMT/UT or British Summer Time. This is the single most common beginner's mistake. A 07:30 high water in a UT table is 08:30 on your watch in summer.
• Height: the predicted height of the water surface above chart datum, usually in metres. It is not the depth under your keel.
• H or L, HW or LW: whether the event is high water or low water. Some tables also mark springs and neaps.

Some tables give a single height for each event. Others add a range or a coefficient. For most coastal days out, the four numbers you need are the next high water time and height, and the next low water time and height.

What the heights are measured from

UK tide tables almost always measure height above chart datum. Chart datum is a low-water level chosen so that the tide seldom falls below it. It is not mean sea level, and it is not the seabed. It is the zero line on the chart.

This matters because a tide height of 1.2 metres does not mean there is 1.2 metres of water everywhere. It means the water surface is 1.2 metres above chart datum. To find the actual depth, add the charted depth to the tide height. If the chart shows 2 metres and the tide height is 1.2 metres, you have about 3.2 metres of water. If the tide height is 4.5 metres, you have about 6.5 metres.

Drying heights work the other way. A rock that dries 1 metre is 1 metre above chart datum. If the tide height is 1.2 metres, there is 0.2 metres of water over it. If the tide height is 0.8 metres, the rock is still uncovered. That is why a low-water height of 0.6 metres is not automatically safe or unsafe. It depends on what is under you.

Springs, neaps and the rule of twelfths

Tides are not the same size every day. Around full moon and new moon, the range between high and low water is at its greatest: these are spring tides. Around the quarter moons, the range is at its smallest: these are neap tides. A table may list MHWS, MHWN, MLWN and MLWS, which stand for mean high water springs, mean high water neaps, mean low water neaps and mean low water springs.

The difference can be huge. A spring high water might be 5.2 metres, while a neap high water might be only 3.6 metres. That is more than a metre and a half of water, enough to change whether a slipway reaches, whether a mooring dries out, or whether a short-cut channel is passable.

Between the table times, you can estimate the height using the rule of twelfths. In the first hour after low water, about one twelfth of the total range comes in. In the second hour, two twelfths. In the third, three twelfths. In the fourth, three twelfths. In the fifth, two twelfths. In the sixth, one twelfth. The same pattern applies on the ebb.

For example, if the range is 6 metres, the tide rises about 0.5 metres in the first hour, another 1 metre in the second hour, and another 1.5 metres in the third. It is a rough tool, but it is far better than guessing.

Planning a trip: a worked example

Suppose a Saturday table for your local port reads: LW 07:30, 0.6 m; HW 13:45, 4.8 m; LW 19:50, 0.9 m. You want to launch a kayak from a slipway that dries 0.8 m. You need at least 1.5 m of water over the slipway, so the tide height needs to be 0.8 m plus 1.5 m, or 2.3 m.

From low water at 07:30 to high water at 13:45, the range is 4.2 m. By the rule of twelfths, the rise after three hours is about 2.1 m, taking the height from 0.6

Photo: Pexels / Pixabay