How long does high tide last?

High tide is a moment; the “stand” is an hour or two

Ask when high tide is and a tide table answers with a minute — 08:52. That minute is the instant the water stops rising and begins to fall, and it lasts no longer than any other minute of the day. The question people are really asking is a different one, and a more useful one: how long does the water stay high enough to count as high tide?

The answer is in the shape of the curve. A tide is close to a cosine, so the level changes fastest halfway between the turns and hardly at all at the turns themselves. Within an hour of the printed high water the sea is still within about seven per cent of the range of its peak; within half an hour, about two per cent. That flat top is the stand of the tide, and it is the honest answer to how long high tide lasts: about an hour if you want the water within a few per cent of its peak, about two hours if a little more will do.

Seven per cent is worth converting into water. Boston has a mean range of about 2.9 metres, so an hour either side of high water the level has come down roughly 20 centimetres from the top, and half an hour either side, about 6 centimetres. On the Bay of Fundy, where the spring range reaches something like 16 metres, that same hour is worth more than a metre. The stand is the same length of time everywhere; it is not the same amount of water. See tidal range for why one coast gets 16 metres and another gets less than half a metre.

How many high tides are there in a day?

On most coasts, two. A semidiurnal coast gets two highs and two lows of roughly equal size every lunar day, one high water following the last by 12 hours 25 minutes. The Atlantic coasts of Europe and eastern North America work this way, and so does most of Africa; Boston is a clean example.

Not every coast does. A diurnal coast gets one high and one low in a lunar day, so the water rises for about twelve hours and falls for about twelve — the Gulf of Mexico is the best-known example, and Pensacola usually prints two rows a day rather than four. A mixed coast, like the Pacific coast of North America, gets two of each, but the two are not the same size: one high is higher and one low is lower. Our guide to the three types of tide sets them side by side.

There is one arithmetic trap in the question. A lunar day is 24 hours 50 minutes and a calendar day is 24 hours, so the tide slips later by about 50 minutes each day and roughly every fortnight a calendar day contains only one high, or only one low. Nothing has gone missing; the second one has been pushed past midnight into tomorrow’s column.

How long between high tide and low tide?

About 6 hours 12 minutes on a semidiurnal coast — half of the 12 hours 25 minutes between one high water and the next. That is the number to hold in your head, and it is close enough that a tide six hours away is a tide you can plan around without a table in front of you.

The six hours are not spread evenly, though, and this is where people misjudge a crossing. The rule of twelfths splits the range into a twelfth in the first hour, two in the second, three in the third, three in the fourth, two in the fifth and one in the last.

Half the water moves in the middle two hours. Practically, that means the two hours around the turn are slow and forgiving and the two hours in the middle are not: on a 4-metre range the sea climbs a metre an hour through the middle of the flood, which is faster than most people walk back across a sandbar. The rule assumes a symmetrical six-hour tide, so treat it as arithmetic for planning rather than a promise; how to read a tide chart works through it on a real table.

When does the water actually stop moving?

Two different things are being asked about here, and a tide table only answers one of them. The level and the current are separate: high water is the moment the level stops rising, while slack water is the moment the current stops running. On an open coast the two are close together. In an estuary, a narrow channel or a river mouth the current turns later than the level does, because water carries on flowing in through the narrows after the level outside has already begun to fall.

That matters if you are anchoring, launching or crossing rather than just watching. Under the bridge at San Francisco (Golden Gate), the stream is what decides whether the trip is easy, and the height alone will not tell you when it turns.

Be clear about what we publish: every station page here predicts the height of the water, not the speed or direction of the stream. The heights come from the FES2022 tide model’s harmonic constants at the nearest ocean point — how we compute them is written out in full. For tidal streams, use a stream atlas or the official current predictions for that channel.

How this changes with springs and neaps

The clock does not change. The interval between one high water and the next is set by the Moon’s orbit, so it stays near 12 hours 25 minutes at springs and at neaps alike, and the stand stays about an hour at both. What changes over the 14.77-day spring–neap cycle is how much water moves in that fixed time.

At springs the range is at its largest, so the same seven per cent is a bigger drop and the middle hours run harder — the water leaves the beach quickly and comes back quickly. At neaps the whole day is lazier: less range, slower streams, and a high water that may not reach the mark you were counting on. The site’s tidal coefficient is the quickest way to tell which kind of day you are looking at before you read a single height.

The tide keeps the same timetable all month. Only the amount of water changes.

Planning around it

Three things follow from all of the above, and they are the ones worth carrying to the water:

• Work the hour either side of the turn, not the exact printed minute. That hour is nearly flat, and it is the margin you have.
• If you need the water to have gone, or to have come back, budget six hours and a bit — and remember that most of it moves in the middle two.
• If you need the current rather than the level, find out when the stream turns where you are. It is not the same moment as high water.

Questions people ask