How to read a tide table

Guide · Tides Pro

You look up tomorrow's tides and get a grid of times and numbers: 04:12, 1.8 m; 10:37, −1.6 m… If nobody has ever walked you through it, it looks cryptic. The good news: a tide table only tells you four things, and once you know what they are, you can read it in ten seconds. Let's take them one at a time.

What a tide table actually tells you

Each row is a day, and each day brings three or four events: high waters (the sea at its highest) and low waters (its lowest), alternating. For every event, the table gives you two numbers:

• The time the sea reaches that maximum or minimum.
• The height of the water at that moment, in metres.

Why do some days have four events and others only three? Because a full cycle takes about 12 hours and 25 minutes — slightly more than half a day. Two highs and two lows usually fit into 24 hours, but every so often one event slips over into the next day.

Heights: why lows can be negative

This is the most common question. In Tides Pro, heights are measured from mean sea level: highs come out positive, and lows can come out negative, because the water genuinely drops below that average level. A −1.6 m is not a bug — it's a generous low tide.

Official tables, on the other hand, usually measure from chart datum, a reference set near the lowest tide you can expect. That's why their numbers are all positive and larger. Neither convention is "the correct one": they are two different rulers measuring the same sea.

The time is the port's, not your phone's

A tide table is always in the port's local time. If you check a port in another time zone — say the Canary Islands from mainland Spain, or Portugal from the UK — that time is not the one on your phone. It's a classic slip that can make you an hour late for low water. Tides Pro automatically uses each port's own time zone, but if you're juggling tables from several sources, always double-check.

Why the tides shift about 50 minutes later every day

If today's high water was at 09:00, tomorrow's won't be at 09:00 — it will be around 09:50. The semidiurnal rhythm of roughly 12 h 25 min follows the Moon, and because the lunar day is longer than the solar day, the whole pattern slides about 50 minutes later each day. That's why this week's "good hour" stops being good next week — and why it pays to check the table every time instead of trusting your memory.

Between high and low: the rule of twelfths

The table gives you the extremes, but the sea doesn't rise in a straight line between them. A classic estimate is the rule of twelfths: split the range into twelve parts and spread the rise (or fall) hour by hour like this: 1/12, 2/12, 3/12, 3/12, 2/12 and 1/12. In other words, during the two middle hours the sea moves half of its entire range. Near the extremes almost nothing happens; at mid-tide, everything happens fast.

That matters if you walk a beach that gets cut off by the rising tide, if you fish or gather shellfish, or if you anchor: the window closes much faster than it seems. And one rule that comes before any arithmetic: always cross-check official sources, mind the local currents, and never turn your back to the sea.

Take the table with you

A tide table is still useful on paper — on the boat, on the fridge at the beach house, or anywhere without signal. Tides Pro generates a printable 12-month report for any of its 90 ports, computed with the FES2022b harmonic model and validated against official IHM predictions, with a median error of about ±10 minutes on the open coast. A whole year of tides in a few sheets.

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