Tide tables look like the dullest document in Britain. A grid of numbers, a few cryptic letters, the occasional asterisk. Yet for anyone who walks the coast, they are the single most useful piece of paper you can carry. Get them right and you will stride out across sand, rock and causeway that is underwater for most of the day. Get them wrong and you may find yourself waving at a lifeboat crew who have better things to do.

This is a short guide to reading a tide table properly, and to using it to plan a shore walk that stays safe from start to finish.

What a tide table actually tells you

A standard UK tide table gives you, for a named port or reference point, the time and height of every high and low water across the day. Heights are usually given in metres above chart datum, which is a low-water baseline rather than mean sea level. That matters: a low water of 0.4 metres is not the same as a low water of 2.1 metres, and on a flat sandy estuary the difference between them is measured in hundreds of metres of walkable ground.

You will also see two other figures. The first is the tidal range, simply high water height minus low water height. A range of 8 metres on the Severn or the Solway means the sea moves fast and covers ground quickly. A range of 1.5 metres in parts of the eastern Irish Sea is gentler but still not to be ignored. The second is the time of high water at the standard port, from which local corrections are applied elsewhere.

• Spring tides occur roughly twice a month, near new and full moon. Ranges are largest, so low water is lower but high water is higher and arrives with more push.
• Neap tides occur between them. Ranges are smallest, which can lull you into a false sense of security on a beach that never quite dries out.
• Local corrections are offsets in time, usually given in minutes, applied to a nearby standard port.

Timing your walk around low water

For most shore walks, the safe window is centred on low water. As a rule of thumb, the sea is at its least intrusive for roughly three hours either side of the low water time, giving you a six-hour window. But that window is not symmetrical in terms of risk. The flood tide is the dangerous one, because it chases you back the way you came and fills the very ground you crossed.

Plan it this way:

• Note the time of low water, then add your estimated walking time plus a generous buffer. Aim to be off the exposed ground at least two hours before the next high water.
• Work backwards from that deadline to decide your start time. If low water is at 09:40 and your route takes three hours, starting at 08:30 gives you a comfortable margin.
• Check the height of the low. A low water of 1.8 metres may leave your intended sandbank still knee-deep, even though the table says "low".
• Check the height of the high. Anything above 4.5 metres in a big-range estuary means the water will come in fast and cover a lot of ground.

The single most common mistake is treating the low water time as the middle of the day's excursion rather than the pivot around which everything else must fit.

Reading a tide table for a specific route

Generic advice only goes so far. What you need is the water level at the particular spot that matters – the neck of a causeway, the lip of a rock ledge, the mouth of an estuary channel you must cross.

Start with the nearest standard port and apply the local correction. Then picture the route in section. Where does the ground dip below the level the tide will reach? For a causeway that floods at 2.5 metres, any high water above that figure closes it entirely. For a sandy bay with a shallow channel, a high water of 3 metres might cut off a headland that a 2-metre tide leaves accessible.

Walk the route on a map before you walk it on the ground. Mark the choke points – the narrow shingle spit, the stepping stones, the beach that narrows against a cliff. These are the places where being twenty minutes late turns a pleasant stroll into a scramble.

Weather, swell and the things tables do not show

A tide table assumes calm conditions. Add a strong onshore wind and a low-pressure system, and the actual water level can run well above the predicted height. A deep Atlantic low can lift sea level by half a metre or more along exposed coasts, and storm surge combined with a spring high is how coastal car parks end up full of seawater.

Swell matters too. On a rocky shore, a big groundswell makes low-water rock platforms dangerous even at the bottom of the tide, because occasional larger sets sweep across them. Wind against tide chops up estuaries and makes wading channels far harder than the chart suggests.

• Check the inshore waters forecast and any coastal flood warnings before you set out.
• If a swell is running, choose a sheltered stretch or a beach walk over a rock platform.
• Never let a calm morning convince you a rising spring tide will behave itself.

Sea angling and the same set of numbers

Anglers have read these tables for generations, and their habits are worth borrowing. Most marks fish best on a particular stage of the tide, often the two hours either side of low water on a beach, or the first of the flood over rough ground. The same window that lets you walk a sandbank safely is the window that puts you in casting range of a gully.

Bait gathering follows the tide too. Ragworm and lug are dug on the biggest spring lows, when the mudflats are exposed furthest. If you plan to dig your own bait before a session, a spring low is your friend – but it also means the flood will return with more energy, so watch your escape route as carefully as any walker.

Above all, write the times down. A tide table in a pocket, with your low water time circled and your turnaround time underlined, has saved more coastal afternoons than any app. The sea keeps an accurate clock. It is only fair that you keep one too.