What a tide table actually tells you

A tide table is not a timetable in the usual sense. It does not tell you when the tide will be at a certain height all day long. Instead, it gives you the two key moments in each tidal cycle: high water and low water. Around the UK coast you will usually see two high waters and two low waters in a 24-hour period, though their times shift later by roughly 50 minutes each day. The table also gives the height of each high and low water, measured in metres above chart datum — a fixed low-water reference level chosen so that most tides never fall below it. That means the numbers you see are not depths of water where you are standing; they are predictions for a specific reference point, often a harbour entrance or a standard port.

For a dinghy sailor, a kayaker, a paddleboarder or even a beachcombing family, these two facts — when and how high — are the foundation of every sensible plan. Get them wrong and you might find yourself hauling a boat across mud, or worse, running aground on a falling tide with nowhere to go.

High water, low water and the tidal range

The difference between the height of high water and the height of the next low water is called the range. A big range means a lot of water moving in and out; a small range means the tide is sluggish and the heights barely change. Around the UK, spring tides — which occur roughly twice a month, just after a full or new moon — produce the largest ranges. Neap tides, around the quarter moons, produce the smallest. In the Bristol Channel, spring ranges can exceed 12 metres; in the eastern Irish Sea or parts of the Solent, a 2-metre range is more typical.

Why does this matter? Because the range tells you how fast the water will move and how much depth you can expect at a given state of tide. A 10-metre spring range means strong tidal streams and rapidly changing depths. A 1.5-metre neap range means gentle currents but also less water over the sandbanks. Knowing whether you are in a spring or neap period is the first thing a sailor checks after the date.

How to read the columns without fear

A typical UK tide table is laid out with a row for each day. You will see columns for the date, the day of the week, and then pairs of times and heights for high and low water. Times are usually given in GMT or BST — check the header, because an hour’s mistake in spring or autumn can ruin your day. Heights are in metres, often to one decimal place.

• Time of high water — when the tide is at its highest at that reference port.
• Height of high water — the predicted level above chart datum.
• Time of low water — when the tide is at its lowest.
• Height of low water — again, above chart datum.

Some tables add a range column, or show sunrise/sunset and moon phases. If you are sailing from a small harbour, your table may be for a standard port such as Dover or Liverpool, with separate pages for secondary ports that list time and height differences. Do not ignore those differences: they can be half an hour and half a metre or more.

The rule of twelfths: estimating depth between tides

Tide tables only give you the extremes. What about two hours after low water? The rule of twelfths is a rough but wonderfully practical way to estimate. Over the roughly six hours between low and high water, the tide rises (or falls) by one twelfth of its range in the first hour, two twelfths in the second, three twelfths in the third, three twelfths in the fourth, two twelfths in the fifth, and one twelfth in the sixth. So if the range is 6 metres, after two hours you have about 1.5 metres of water above low water — 1/12 plus 2/12 of 6 metres. It is not exact, but it is close enough for anchoring, launching a boat, or deciding whether to walk across a causeway.

Always add a safety margin. Weather matters: a sustained low pressure system can raise water levels by 30 centimetres or more, whilst high pressure can lower them. Strong onshore winds can pile water into estuaries, and offshore winds can push it out. The tide table is a prediction, not a promise.

Secondary ports and local quirks

Most UK coastal sailors do not sail from a standard port. They sail from a creek, a bay, or a small harbour that has its own tidal personality. That is where secondary port tables come in. They give you a time difference — say, high water at your port is 45 minutes later than at the standard port — and a height difference, perhaps plus 0.3 metres at high water but minus 0.1 metres at low water. Some ports also have a drying height or a sill that you must cross within a certain window. Local almanacs and harbour authority notices often add notes about sandbars that shift after winter storms.

Do not assume that a tide table for one place works for another just because it is nearby. Estuaries, headlands and narrow channels can distort the tide so much that two ports five miles apart have completely different timings.

Planning a day afloat or ashore

Start with your activity. Launching a dinghy from a slipway? You want enough water to float the trailer, but not so much that the slip is underwater and the current is ripping past. Walking to a tidal island? You need low water plus a two-hour window either side, and a clear exit before the flood. Anchoring for lunch? Check the range and the wind direction — you will swing differently on a spring ebb than a neap flood.

Write down your plan: high water time, low water time, range, and your safety margins. Tell someone ashore. Take a paper copy of the tide table, not just a phone app, because batteries die and signals drop in remote coves. And remember that the sea does not care about your schedule. Reading a tide table well is simply learning to work with the rhythm of the coast — and that rhythm, once you feel it, makes every day on the water calmer, safer and more enjoyable.