The four things on every line
Every tide page on this site lists each day separately, in Irish local time. For each day you get the times of high water and low water and a height next to each one.
- Time. The moment the water stops rising (high water) or stops falling (low water). It is already in local time, so summer time is handled for you.
- High or low. Most Irish days have two of each. Every so often a day has only three events, because one of them slipped past midnight into the next day.
- Height. How far the water is above (or below) a fixed zero line, in metres. On our pages the zero is Ordnance Datum Malin. More on that below, because it is the part that confuses people most.
- Range. Not printed on every line, but the 30-day calendar shows it: the difference between the day’s highest high and lowest low. Big range means spring tides, small range means neaps.
A worked example: Dublin, Sunday 1 November 2026
These are the Marine Institute predictions for the Dublin Port gauge for that day:
| Time | Tide | Height (OD Malin) |
|---|---|---|
| 03:55 | High water | 1.2 m |
| 09:20 | Low water | −1.0 m |
| 16:20 | High water | 1.4 m |
| 22:15 | Low water | −1.3 m |
Read it like this. The sea is high before dawn, drains out across Sandymount and Dollymount through the morning and is lowest at about 9:20. It then rises all afternoon to its highest at 16:20, just before the sun sets (sunset in Dublin that day is about 16:53). The difference between the afternoon high and the morning low is 2.4 m. That is a middling tide for Dublin: big springs there reach about 3.7 m, quiet neaps under 2 m.
So a walk on the strand fits between roughly 8:20 and 10:20, and a swim at a shelving beach suits the hour or so before 16:20. Our station and beach pages work this out for you in the “Best times this week” table, cut to daylight.
Why the heights are negative
The zero on our pages is Ordnance Datum Malin: the height of mean sea level measured at Malin Head between 1960 and 1969, which Ordnance Survey Ireland uses as zero for every height on its maps. It sits close to today’s average sea level. So a low water is usually below it, and the number is negative. That does not mean the harbour is empty. It means the water is 1.0 m below the line that maps call zero.
Dublin Port’s own printed tables for the same day give the afternoon high as 3.78 m and the morning low as 1.43 m, because they measure from chart datum, a zero set near the lowest tide possible. Add about 2.4 m to our Dublin numbers and you land within a few centimetres of theirs. The chart datum guide has the offset for each major port.
How fast the water moves: the rule of twelfths
The tide does not rise at an even pace. It starts slowly, speeds up in the middle and slows again near the top. Sailors estimate the in-between heights with the rule of twelfths: split the time from low to high into six equal parts, and the water rises 1, 2, 3, 3, 2, 1 twelfths of the range in those parts.
Apply it to the Dublin afternoon above. Low water 09:20 at −1.0 m, high water 16:20 at 1.4 m: a rise of 2.4 m over 7 hours, so each part is 70 minutes and one twelfth is 0.2 m.
| By | Rise in that part | Height |
|---|---|---|
| 10:30 | 1/12 = 0.2 m | −0.8 m |
| 11:40 | 2/12 = 0.4 m | −0.4 m |
| 12:50 | 3/12 = 0.6 m | 0.2 m |
| 14:00 | 3/12 = 0.6 m | 0.8 m |
| 15:10 | 2/12 = 0.4 m | 1.2 m |
| 16:20 | 1/12 = 0.2 m | 1.4 m |
Two things follow. First, half the whole rise happens in the middle third of the flood, between about 11:40 and 14:00 here. That is when sandbars cover and channels fill behind walkers. Second, the hour after low water feels deceptively quiet. People walk out in that hour and are still out when the fast middle of the flood arrives. The rule is an approximation and it works poorly where the tide has an odd shape, such as Wexford and Arklow, where the range is small and the curve flat.
Springs, neaps and the 50-minute shift
Tides follow the moon, and the moon comes back over the same spot about every 24 hours 50 minutes. So each day the tides arrive roughly 50 minutes later than the day before. The step is not even: in the Dublin example the morning low moves from 9:20 on Sunday to 10:40 on Monday, 80 minutes later, because the step is longer around neaps and shorter around springs. That is why the best day for an evening swim moves through the week.
Around full moon and new moon the sun and moon pull in line and the range is biggest: these are spring tides, nothing to do with the season. A week later, at the quarter moons, the range is smallest: neap tides. Our spring and neap tide page lists the next dates.
What a tide table does not tell you
The predictions cover the moon and the sun only. The Marine Institute says plainly that storm surge from air pressure and wind is not included. A deep Atlantic low raises the sea about 1 cm for every hectopascal of pressure below normal, so a 980 hPa storm can add around 30 cm, and an onshore gale piles more on top. In settled weather trust the heights. In a storm treat the predicted high water as a floor and check the Met Éireann sea area forecast. The guide on why tide times differ goes into this.