Tide Table — High and Low Water Times, Tide Chart and Coefficient
High and low water for 500 ports in 85 countries, with heights above chart datum, a chart you can drag to read the water level at any moment, and the two numbers most tide sites leave out: whether today is a spring or a neap, and the tide coefficient. The whole prediction runs in this page — the harmonic constants are built in, so there is no server to ask and nothing to upload.
| Day | High water | Low water | Coef | Tide |
|---|
What a tide table is actually telling you
A tide table lists the moments the water stops rising and starts falling, and vice versa — high water and low water — with the height it reaches. Most coasts get two of each a day, roughly every 12 hours and 25 minutes, which is why high water slides about 50 minutes later each day. Some places get only one cycle a day; a few, like parts of the Gulf of Mexico and the South China Sea, alternate.
The heights are not depths and they are not distances from the beach. They are metres above chart datum — a fixed level, chosen so the sea almost never falls below it, that nautical charts measure their depths from. So if the chart says 2.4 m at a spot and the table says 3.1 m, there is about 5.5 m of water there. That is the whole reason chart datum exists, and it is why every number on this page is positive.
What a tide table cannot tell you is what the water will actually do. It is an astronomical prediction — Moon and Sun only. Add a deep low-pressure system and an onshore gale and the real level can sit half a metre above the prediction for a day, or several metres in a surge. That gap is why the warning above this section is there and why official tables carry the same one.
Spring and neap tides, and why your beach looks different every week
The Moon raises the tide; the Sun raises a smaller one, a little under half the size. Twice a lunar month, at new moon and full moon, the two line up and reinforce each other — that is a spring tide, with the highest highs and the lowest lows of the fortnight. At the quarter moons they pull at right angles and partly cancel: a neap tide, with everything compressed toward the middle.
“Spring” has nothing to do with the season. It is the older sense of the word, as in a spring of water — the tide springs forth.
The effect is large. At Dover a spring range is roughly twice a neap range, so the same beach can lose or keep hundreds of metres of sand depending on the week. If you are planning anything that depends on how far the water goes out — a walk to an island, launching off a slipway, a dive on a wreck — the spring/neap label on this page matters more than the exact time.
Springs lag the new and full moon by a day or two, and the lag is different at every port. Rather than assume it, this page classifies each day by comparing that day's range against the biggest and smallest range the port itself reaches over the surrounding lunar month. It means the labels stay correct on coasts where the usual rule of thumb breaks down.
The tide coefficient, and how to read it
French and Belgian tide tables print a number between 20 and 120 next to each tide. It is the coefficient de marée, and it compresses the whole spring/neap question into one figure: the day's range as a percentage of the port's average spring range.
| Coefficient | Means | In practice |
|---|---|---|
| 20–40 | weak neap | the water barely moves; poor for anything that needs a big drop |
| 45–70 | average | ordinary conditions |
| 70–95 | strong spring | good low water; strong tidal streams |
| 95–120 | very strong | the biggest tides of the year, around the equinoxes |
95 is the point where French harbourmasters start paying attention, and 100+ is what fills the causeway at Mont-Saint-Michel and empties the oyster beds in the Bay of Arcachon. Above roughly 110 you are looking at a handful of days a year.
Strictly the coefficient is defined at Brest. Because it is a ratio rather than a height, it carries over to any port using that port's own mean spring range, which is how it is computed here.
Where these predictions come from, and how good they are
Tides are predicted by harmonic analysis. Years of measured sea level at a port are decomposed into a few dozen sine waves, each at a frequency fixed by astronomy — the Moon's orbit, the Sun's, the tilt and wobble of both. Each wave gets an amplitude and a phase unique to that port. Add them back together for any future date and you have the tide.
The constants used here come from two public sources: NOAA for the United States and its territories, and TICON-4, derived from the GESLA-4 global sea-level record, for everywhere else. They are bundled into this page for 500 ports, filtered to only those published under a public-domain or CC BY licence.
Two honest caveats. First, only constituents above 3 mm are shipped, to keep the page a reasonable size — measured against the full data, that shifts a predicted height by about 3 mm typically and under a centimetre at the 95th percentile, which is invisible next to the 0.1 m the table shows. Second, the quality of any harmonic prediction depends on how long and how clean the original record was; a port with two years of data is not as well characterised as one with fifty.
Neither of those is the main source of error. Weather is. A prediction that is perfect astronomically can still be half a metre out because the air pressure is low and the wind is onshore.
Reading the chart and getting the level at a moment
The curve is the water level through the day. High and low water are marked with their times and heights; the shaded bands are the hours between sunset and sunrise, and the dashed red line is now.
Move along the curve and it reads out the level at that exact moment. That is usually the real question — not “when is high water” but “will there be enough water at half past four”. It works by touch as well.
A useful shortcut for doing it in your head is the rule of twelfths: in the six hours between low and high water, the tide rises 1/12 of its range in the first hour, then 2/12, 3/12, 3/12, 2/12 and 1/12. Half the movement happens in the middle two hours. It assumes a smooth semidiurnal curve, so it works well in the English Channel and badly in a port with a distorted tide — compare it against the curve here and you can see at a glance whether it applies where you are.