How to read a tide chart

What a tide chart shows

“Tide chart” is used for two things, and a station page here carries both: the table of high and low waters, and the curve drawn through them. Start with the table. Most coasts get two highs and two lows a day, listed with a local time and a height above the datum. The difference between a high and the adjacent low is the range for that tide, and it changes from day to day with the spring–neap cycle.

Each row is one turn of the water and carries three things: whether it is a high or a low, the minute it happens, and how high the water gets. A semidiurnal day looks like this, with the turns about 6 hours 12 minutes apart and each high 12 hours 25 minutes after the last.

Four rows is the normal case — Boston prints four on most days. Some coasts print two, because they get one high and one low in a lunar day, and on a mixed coast the two highs are not the same size, so the row you want is not always the next one down. Our guide to the types of tide covers which coast is which.

Reading the curve

The table gives you four moments; the curve gives you the rest of the day, and its shape is the message. Every tide leaves a turn slowly, runs hardest halfway between two turns — about a quarter of the range in that middle hour — and then flattens out into the next one. Equal steps along the bottom of the chart are not equal steps of water.

That is worth reading before any single number. Where the line is steep, being half an hour late costs you real depth; where it is flat, half an hour costs you almost nothing. The high and low badges sit on the turns, so you can see at a glance whether the moment you care about falls on a steep stretch or a flat one, and the marker for the current time shows where the day has got to.

On any station page — Plymouth, say — the chart zooms in to six hours when you want the next tide in detail. A week at a time — the view for choosing which day to go — is in the signed-in tool, which is free.

The rule of twelfths

Between low and high water, the sea rises about 1/12 of the range in the first hour, then 2/12, 3/12, 3/12, 2/12 and 1/12. Half the water moves in the middle two hours. It is an approximation that assumes a six-hour, symmetrical tide, but on most coasts it is close enough to plan an anchorage or a walk across a sandbar.

Work it on the sample table above. The range is 4.0 metres, so a twelfth is about 0.33 metres. Two hours after the 02:40 low the water has come up three twelfths — a metre. Halfway between the two turns, at 05:46, it is at half tide and moving at its fastest: the rule gives it a full metre in the third hour alone, exactly as much as the first two hours managed between them. If you need a number for a particular minute, though, read it off the curve; the twelfths are for doing in your head on the beach.

Why the times slide each day

The tide follows the Moon, whose day is 24 hours 50 minutes, so high water arrives roughly 50 minutes later each day. Over a week that is nearly six hours — a tide that was convenient on Monday morning is inconvenient by Friday.

Two things follow. Yesterday’s times are never today’s, so a chart is worth re-reading rather than remembering; and because the lunar day is longer than the calendar day, roughly every fortnight a date holds only one high water, or only one low. Nothing is missing from the table — the other one has slipped past midnight into the next day’s rows. How the Moon affects the tides works through where the 50 minutes comes from.

Heights, datums and negative numbers

A height is meaningless without the level it is measured from. That level is the datum, and different publishers use different ones: nautical charts use a low datum such as LAT or MLLW, chosen so that the sea very rarely falls below the depths printed on the chart. The heights here come from the FES2022 global tide model and are relative to mean sea level, which sits in the middle of the tide rather than under it — so on this site low waters print as negative numbers.

A negative height is not an error and not a dry seabed; here it is the ordinary case. Our zero is the local average sea level, so the water is below it for part of every day, and a low water is simply reported as the distance below that average — a −1.2 m low is 1.2 m under the average, not 1.2 m under the seabed. What it does mean is that you cannot add our number to a charted depth: the two are counted from different zeros, and mixing them will flatter the depth you think you have. For a charted depth, use the official table for that port. What is chart datum lays the levels out side by side.

One more thing worth knowing about our stations. Where a station is a real tide gauge, our catalogue carries the names of its official datums, and the page badges it as a gauge because a measured record exists there — but the prediction we draw is still the model’s, relative to mean sea level. Gauge vs. interpolated stations explains what that badge does and does not promise, and how we compute the numbers is written out in full.

Time zones, DST and the traps

A table is only right if it is in the time zone you are standing in. Ours are always in the station’s local time, with daylight saving already applied. The other classic trap is comparing two sources on different datums (see our chart-datum guide) and concluding that one of them is wrong.

The rest of the list is short, and every item on it has caught somebody out:

• Reading a chart for the wrong place. A station up an estuary can turn noticeably later than the one at its mouth.
• Assuming the second high of the day matches the first. On a mixed coast they differ, sometimes by a lot.
• Planning from a printed sheet a week old. The times have moved on by nearly six hours.
• Treating the prediction as a measurement. It is astronomy: air pressure alone shifts the level about a centimetre per hectopascal, and a storm with an onshore wind can shift it half a metre or more, and change the timing with it.

Reading a tide chart for fishing

A tide chart tells an angler two useful things. The first is which way the water is going: rising towards a high, or falling towards a low. The second is how hard it is going — and that comes out of the shape rather than the rows, because half the range moves in the middle two hours of every tide, while the hour either side of each turn barely moves at all. How long high tide lasts works that hour out in detail.

Whether a flood beats an ebb is a question the astronomy cannot answer, and we are not going to pretend otherwise: it depends on the mark, the species and where the food is being carried. What the chart does let you do is choose deliberately — fish the same state of tide twice and compare, rather than blaming the day.

Alongside the tide, every station page marks the solunar windows: two major periods of about two hours when the Moon is overhead or underfoot, and two minor ones of about an hour at moonrise and moonset. That is a rule of thumb from 1926, not a measured effect, and we present it as one — solunar theory explains where it comes from. For picking the day rather than the hour, the tidal coefficient ranks the whole month at a glance.

Where the curve beats the table

A table gives you four points; the curve gives you every minute in between. On any station page you can scrub the chart to read the predicted height at an exact time, switch to the table view when you want the numbers, and export the month as CSV or print it for the boat.

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