How far ahead can I extrapolate from one high water?

Two or three days keeps the times within about fifteen minutes on most coasts. Beyond that the small irregularities the constant interval ignores — the moon's changing declination and distance — accumulate, and a week out you can be half an hour off. Heights degrade faster than times, because the spring and neap cycle changes the range substantially within a fortnight.

Where does the rule of twelfths come from?

It is a whole-number approximation of a sine curve. If the tide rises and falls sinusoidally over six hours, the fraction risen after each hour is 6.7, 25, 50, 75, 93.3 and 100 per cent; the twelfths give 8.3, 25, 50, 75, 91.7 and 100. The middle four hours are almost exact and the first and last are a point or two out — close enough to do in your head on a boat, which is what it was for.

Does the Mediterranean have tides?

Yes, but small ones — typically a few tens of centimetres, against several metres on the Atlantic coast — because the basin is nearly enclosed and too small for the tidal wave to build. The northern Adriatic is the exception, where the shape of the gulf amplifies the range enough to matter in Venice. On a coast with a range of twenty centimetres, the rule of twelfths is arithmetically fine and practically pointless: the wind will move more water than the moon.

Predicting Tides From a Single Known High Water

Published 10/15/2026 · 6 min read · Everyday calculators

Lena Hoffmann — Science & education writer at OneKitly

Mathematics · Physics

Checked against 3 sources

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In short

On a semidiurnal coast — the Atlantic pattern, two highs and two lows a day — high water repeats every 12 hours 25 minutes and low water falls 6 hours 13 minutes after each high. So a high water at 06:30 gives the next at 18:55, then 07:20 the following morning: the whole cycle slides about 50 minutes later each day, because it follows the moon rather than the sun. To estimate the height between two turns, use the rule of twelfths: of the total range, the water moves 1/12 in the first hour, 2/12 in the second, 3/12 in the third and fourth, 2/12 in the fifth and 1/12 in the last. Half the range moves in the two middle hours alone.

One high water and the 12 h 25 min lunar interval give you every high and low for days. The rule of twelfths then tells you how much water has moved at any point in between — and both stop being reliable at exactly the same place.

Why the interval is 12 h 25 min and not 12 h

The tidal bulges are anchored to the moon, and the moon does not stand still while the Earth turns. By the time a point has spun once under the moon, the moon has moved on along its orbit and that point needs about fifty extra minutes to catch up. That is the lunar day: 24 h 50 min instead of 24 h. Halve it and you get the 12 h 25 min between successive high waters on a semidiurnal coast.

That fifty-minute daily slip compounds fast. Over a week it amounts to nearly six hours, which is why a beach that was uncoverable at breakfast on Monday is under water at breakfast the following Sunday. Anyone who plans a walk to a tidal island from last week's timings is not slightly wrong; they are half a cycle wrong.

Three patterns, and how to tell which one you are on

Semidiurnal coasts get two highs and two lows a day of roughly equal height: most of the Atlantic seaboard, from Portugal up through Brittany to the North Sea. Diurnal coasts get one of each — parts of the Gulf of Mexico and of South-East Asia — and their interval is the full lunar day of 24 h 50 min. Mixed coasts, common on the Pacific side of North America, get two highs a day of visibly different height, a higher high and a lower high, and the same for the lows.

The calculator asks you which of the three you are on, and it should: getting this wrong is the one input error that produces confidently wrong answers rather than obviously wrong ones. Pick diurnal on a semidiurnal coast and it will hand you one tide a day at plausible-looking times, with the other one silently missing. The cheapest way to check is to look at two consecutive days of any real tide table for your port and count the lines.

What this method cannot know

The interval is a constant; the real ocean is not. Tidal range swells at new and full moon, when the sun and moon pull together, and shrinks at the quarters — the spring and neap cycle, worth a factor of two on many coasts. The moon's declination and its elliptical orbit modulate the interval slightly from one tide to the next. And the shape of a bay can do far more than any of that: the Bay of Fundy resonates with the tidal period and turns a metre of open-ocean range into more than sixteen.

Weather adds the last layer and it is the one that hurts. A deep low raises sea level by roughly a centimetre per hectopascal of pressure drop, and a sustained onshore wind piles water against the coast on top of that. A storm surge can add a metre to a predicted height, which is why every official tide table is a prediction of the astronomical tide alone and why the tool says, in as many words, to consult one before navigating. Use this method to know when, not to decide whether.

The rule of twelfths over a 12 ft range
Hour	Share of the range	Water moved this hour	Total moved
1st	1/12 — 8.3 %	1 ft	1 ft
2nd	2/12 — 16.7 %	2 ft	3 ft — 25 %
3rd	3/12 — 25 %	3 ft	6 ft — 50 %
4th	3/12 — 25 %	3 ft	9 ft — 75 %
5th	2/12 — 16.7 %	2 ft	11 ft — 91.7 %
6th	1/12 — 8.3 %	1 ft	12 ft — 100 %

Tide Time CalculatorEstimate the day's high and low tide times from a known reference tide and pattern.Try the tool →

Frequently asked questions

How far ahead can I extrapolate from one high water?: Two or three days keeps the times within about fifteen minutes on most coasts. Beyond that the small irregularities the constant interval ignores — the moon's changing declination and distance — accumulate, and a week out you can be half an hour off. Heights degrade faster than times, because the spring and neap cycle changes the range substantially within a fortnight.
Where does the rule of twelfths come from?: It is a whole-number approximation of a sine curve. If the tide rises and falls sinusoidally over six hours, the fraction risen after each hour is 6.7, 25, 50, 75, 93.3 and 100 per cent; the twelfths give 8.3, 25, 50, 75, 91.7 and 100. The middle four hours are almost exact and the first and last are a point or two out — close enough to do in your head on a boat, which is what it was for.
Does the Mediterranean have tides?: Yes, but small ones — typically a few tens of centimetres, against several metres on the Atlantic coast — because the basin is nearly enclosed and too small for the tidal wave to build. The northern Adriatic is the exception, where the shape of the gulf amplifies the range enough to matter in Venice. On a coast with a range of twenty centimetres, the rule of twelfths is arithmetically fine and practically pointless: the wind will move more water than the moon.

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