How to Read a UK Tide Table: A Beginner's Guide to Times, Heights and Curves
A tide table is one of the least glamorous documents in coastal life: a grid of numbers, a couple of abbreviations, nothing that looks remotely like weather. Yet it answers the question that decides most days at the shore — will there be water where I want to be, and when will it be there?
Once you can read four things on the page — the columns, the datum, the time zone and the curve between the numbers — the rest is straightforward arithmetic. You do not need to be a sailor. You just need to know what you are looking at.
A UK tide table gives predicted times and heights of high and low water for one specific place. That last part matters. A table for Dover is not a table for Brighton, and neither of them will be much use at Whitby.
For most of the British coast the tide is semi-diurnal: two high waters and two low waters in roughly 24 hours and 50 minutes. That extra 50 minutes is why high water slides about an hour later each day, and why the same beach walk can feel completely different on a Saturday and a Sunday. Some spots behave differently — parts of the Solent have a well-known double high water, where the level hangs around before it starts to fall, and some northern Scottish waters lean towards a single high and low each day. Local shapes and shallow seabeds all leave their fingerprints on the pattern.
Formats vary between publishers, but a typical table row contains the same handful of pieces:
A row reading 07:42 4.2 H means high water is expected at 7.42am, and the level will reach 4.2 metres above chart datum. If the same day shows 13:58 0.6 L, the range between them — 3.6 metres — is how much water moves in and out.
Heights in a tide table are not measured from the seabed, and they are not measured from mean sea level. They are measured from chart datum, which around the UK is generally Lowest Astronomical Tide, or LAT. Put simply, it is the lowest level the water is expected to reach under normal astronomical conditions.
This is why a harbour that dries out at low water can still show a positive height, and why you will occasionally see a negative one on a big spring tide or in a table for a shallow estuary. Chart datum is also the reference for the depths printed on nautical charts, which gives us the only formula worth memorising:
Actual depth of water = charted depth + height of tide
Say a chart shows 1.8 metres over a sandbank, and the tide table says 1.4 metres at the time you plan to cross. There is 3.2 metres of water over the bank — enough for many small boats, not enough for others. Run the same sum at low water with a height of 0.2 metres and you have 2.0 metres. Same bank, same chart, very different day.
Two traps catch beginners again and again. The first is time zone. Many official tables are published in GMT or UT, while websites and apps often show local time, which means British Summer Time from late March to late October. Always find the note that says which one you are reading. An hour's error is the difference between a comfortable crossing and a call to the coastguard.
The second is location. If your destination has no table of its own, you will be using a secondary port: a standard port's predictions plus a set of time and height corrections. These work well when applied properly, but they are approximate, and shallow estuaries, tidal rivers and narrow channels can throw up local quirks that no correction table captures.
A tide table tells you the peaks and troughs. It says nothing about how the water got there — and that is where things get interesting. The tide does not rise at a steady rate. It moves fastest around mid-tide and slows to a crawl near high and low water.
A useful rough tool is the rule of twelfths. Over a six-hour rise, the water gains roughly one-twelfth of the range in the first hour, two-twelfths in the second, three in the third, three in the fourth, two in the fifth and one in the sixth. With a 4.8-metre range, each twelfth is 0.4 metres, so the cumulative rise after low water looks like this:
Half the water arrives in the middle two hours. That is why a beach that seems to have plenty of room can be cut off surprisingly quickly, and why "high water is at four, so we're fine until three" is not a safe assumption. The rule is an approximation, and steep-sided estuaries and shallow bays will not obey it neatly, but it is far better than guessing.
Tide predictions are astronomy, not weather. Low pressure can raise or lower water levels well beyond the predicted figure, and a strong onshore wind can push water up a funnel-shaped estuary or hold it back. Swell, river flow after heavy rain, and local currents all change the picture on the ground.
So treat the table as a very good starting point rather than a guarantee. If your plans depend on precise depth or a tight timing — a harbour entrance, a boat, a causeway crossing — speak to the harbour authority, check the relevant almanac and take local advice. In an emergency on the coast, call the coastguard.
Before any coastal trip, do this once and you will rarely be caught out. Note the two high waters and two low waters for the day, mark the times when the water is rising and falling, and pick your window around them rather than around your own convenience. Then build in slack. The sea keeps its own schedule, and the best days on the coast belong to people who read it in advance and still leave room for it to surprise them.
Photo: JillWellington / Pixabay
