Read tide heights, datum, slack water and the rule of twelfths — a practical guide to timing tidepools, shore dives and beach walks safely.
Every good beach day starts the night before, at a table of small numbers. Tide tables look like they were designed to be ignored — columns of heights, am and pm, plus and minus — but they answer the three questions that actually decide your day: when can I see the reef, when is it safe to walk there, and when do I need to leave?
This guide covers what tide tables measure, how to read the numbers you find, and how to plan around them for tidepooling, shore diving and beach walking. Ten minutes of reading now buys you a full, unhurried day later.
A tide table is a prediction of water height, in time. That's it. Every row gives you:
Tides run on a roughly 12-hour and 25-minute cycle from high to high, so most coasts get two highs and two lows a day. Along the North American west coast, the two highs are usually unlike each other — one much higher than the other — which is why your table shows four events, not two, and why "low tide" alone can mean two different things in the same day.
The numbers also come as predictions, not measurements. They're computed from astronomy — the pull of moon and sun — plus years of recorded water at that station. Two things predictions don't fully capture: barometric pressure (a strong high-pressure system presses the water down; lows let it rise) and wind (a stiff onshore gale can pile water well above prediction). On most days the difference is negligible; on storm days, treat your low as mysteriously lower-limited and walk with extra margin.
Find a table for your nearest prediction station. In the US, NOAA publishes them free; harbour offices, marinas and many coastal pubs carry printed ones. A typical row might read:
03:47 Low -0.6 ft — the morning's "minus" tide
10:52 High +6.1 ft — the day's better high
16:38 Low +2.9 ft — the shallow, afternoon low
Notice three features of those rows.
One: the datum. Heights are measured against a local reference line, and on much of the west coast that line is Mean Lower Low Water — the average of the lower of each day's two lows. Heights above it are positive; heights below it are negative, which is what the "minus" in "minus tide" means. A negative tide is special: the water has dropped below a reference line the sea itself kept as a floor, exposing reef that only appears a few times a year.
Two: tide direction between events. The table gives you four points and leaves the rest of the curve to you. A rough way to fill the gap is the rule of twelfths: over each six-hour tide, the water moves about 1/12 of its total range in the first hour, 2/12 in the second, 3/12 in the third and fourth (the fastest middle of the tide), then 2/12 and 1/12. You don't need the maths in the field; just remember the tide runs slow at the ends and fast in the middle.
Three: time zones and stations. Tables are published for primary stations; nearby spots get corrections — "at Pillar Point, add 14 minutes, subtract 0.2 ft." If your beach isn't the named station, apply its local correction (they're in the same publication) or use the closest station's table but expect to be half an hour off.
Slack water is the brief stillness when the tide turns and the current pauses — a word borrowed from sailors and divers, who plan their whole day around it. The received wisdom says slack coincides with high and low water. On open-ocean coasts, more or less true. Inside bays, estuaries and narrow passages, often not close at all.
The reason is plumbing: rising water at the headlands keeps pushing water into a bay for a while after the tide outside has started to fall; the current only goes slack when the bay has caught up. Strong channels can lag by an hour or more. For real passages (Puget Sound narrows, Deception Pass, any river mouth), find a current table, which is a separate publication that predicts slack, flood and ebb explicitly. Our guide to reading marine forecasts covers the wind-and-swell side of that pair.
For a shore dive or a snorkel at a kelp cove, plan entry on the flood in the last hour before high tide: water is rising over the shallows, viz is usually best, and the current, if any, is carrying you gently homeward.
The tidepool day. Aim to arrive one to two hours before the low, explore as the reef drains, and leave past the low point as water returns. That timing gives you the longest window on the low-water ledges without ever being chased. On a minus tide you may find an hour of reef that behaves like open sea urchins and abalone country; on a positive low, the pools stay ankle-deep and busy and that's fine too. Watch the occasional bigger "sneaker" set — long-period swell can arrive without warning and sweep ledges even at low tide.
The long walk. Some shoreline routes — around-headland beach walks, bluff-base trails — are only passable at low water. Plan these on the ebb, not the flood: start shortly after the high and walk with falling water behind you, so the day never closes the door behind you. If a route is labelled "passable four hours around low tide," take the conservative reading of that offer.
The dive or snorkel. Clear water tends to arrive with the flood tide's cleaner ocean water; the outgoing tide can carry suspended sediment in, especially after rain. Enter in the last hour of the flood, dive into the slack ride out, ride out in the first of the ebb. New to rock entries? Read our guide to rocky shore entries and exits before your first face-full of seaweed.
Tide heights swing through a fortnight cycle driven by the moon. Around the new and full moon, sun and moon pull in line, and the tide goes higher than average at high and lower than average at low — these are spring tides (the spring is from "spring forth," not the season). At the quarter moons, the pulls partly cancel, giving neap tides with modest highs and shallow lows.
For tidepooling, springs are gold, and the very biggest ones — the king tides we explain here — expose reef that exists as land for only a few days a year. For diving in currents, neaps are kinder. For everything else, consult the numbers: the day with the deepest negative low of the year is usually on your table three months in advance.
By convention some height references — Mean Lower Low Water on the US west coast, for example — sit above the occasional lowest tides. Days when the water falls below that floor read as negative, and they're the ones that expose spectacular low-tide reef. The Japanese and some other systems use a different datum, so check what yours measures.
Usually good to within a few centimetres in average weather. Low pressure and onshore wind can push water well above prediction; high pressure and offshore wind below it. In those conditions, stay conservative about exposed routes and don't rely on the predicted "safe" window.
In a six-hour tide, the water moves slowly in the first and last hour (about an eighth of the range each) and quickly through the middle four hours. So the hour before your target is the time when things change fastest — arrive before that hour is near its end, not after.
With a correction, yes. Each minor spot has published time and height adjustments from a primary station; apply them and you're usually within tens of minutes. Without the correction, you could be off by an hour or more — risky on a shore with limited exit points.
No. Tables predict the astronomical tide only. For wind, swell, surf and thunderstorm risk, pair them with a proper marine forecast — see our marine forecast guide.
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