What is the tidal coefficient? The 0–120 scale, explained

Guide · Tides Pro

Open a tide table for Spain, Portugal or France and, next to the high and low water times, you'll spot a number between 0 and 120. That's the tidal coefficient — and once you know how to read it, it becomes the first thing you check: at a glance, it tells you how much the sea is going to move that day.

In this guide we'll cover what the coefficient actually is, where it comes from, what spring and neap tides mean in the water, and how to use it to pick the best day for fishing, shellfishing or the beach.

What the tidal coefficient actually is

The tidal coefficient is a unitless index on a scale of 0 to 120 that sums up the "strength" of each day's tide. The formula comes from Spain's Instituto Hidrográfico de la Marina (IHM) and is built from the two main components of the tide: M2, driven by the Moon, and S2, driven by the Sun.

When the Moon and Sun pull on the ocean along the same line — at new moon and full moon — their effects add up and the coefficient rises. When they sit at right angles — first and last quarter — they partly cancel out and the coefficient drops. That's why the coefficient follows the lunar month, rising and falling twice per lunation, like a slow breath.

One important detail: the coefficient doesn't measure metres. It's a proportion relative to the average tide, and the same value applies along the whole coast. On a day when Santander has a coefficient of 90, Vigo has it too; what changes is how many metres that 90 translates into at each port — more on that below.

Neaps, average tides and springs: the three bands

In practice, you read the coefficient in bands, and each band describes a different sea:

• Neap tides (coefficient below 40). Little difference between high and low water. The sea barely breathes: the shoreline hardly moves, currents are gentle and low tide uncovers little seabed.
• Average tides (40 to 79). The most common situation of the month: a clearly noticeable tide, without extremes.
• Spring tides (80 or more). A big range: higher highs, lower lows and noticeably faster currents, because the same water has to flow in and out in the same amount of time.

Springs always arrive one or two days after the new and full moon — a lag known as the "age of the tide". And remember that on the Atlantic Iberian coast the tide is semidiurnal: a full cycle takes about 12 hours and 25 minutes, so most days bring two high waters and two low waters, each day running a little later than the one before.

Using it to plan fishing, shellfishing and beach days

• Shellfishing. You want the lowest low water possible — that means a high coefficient. On spring days, low tide uncovers rock and sand that stay submerged the rest of the month.
• Fishing. Moving water stirs up food and switches on the predators: many shore anglers look for high coefficients and fish the hours of strongest flow. With neaps, on the other hand, it's easier to anchor, float-fish or learn the ropes. There's no universal rule — note what works on your stretch of coast and cross-check it against the coefficient.
• Beach and rock pools. With springs, a beach changes face within hours — huge at low water, tiny at high water — and the rock pools get refreshed. With neaps, the shoreline barely moves: ideal if you want a "stable" beach for the day.

And a serious note: with high coefficients the sea rises fast and currents pick up. Always check official sources before heading out, learn about the currents in your area, and never turn your back on the sea — especially on rocky shores.

Same coefficient, different tide: every port has its own amplitude

The coefficient is a multiplier, not a measurement. It tells you how much the tide is amplified relative to its average, but every port has its own amplitude, shaped by the coastline, the continental shelf and the resonance of the basin. That's why the same coefficient produces very different ranges on the Cantabrian coast, in the Gulf of Cádiz or inside a ría.

The practical takeaway: don't compare metres between ports — compare coefficients. And when you need the actual height at your port, look at its tide curve: it's the combination of the day's coefficient and the local amplitude that tells you how much water you'll have under the keel, or how much beach will be left uncovered.

Where to see the coefficient in Tides Pro

In Tides Pro, the day's coefficient is always one tap away: it sits next to each day's tide curve, with its neap, average or spring band, across the app's 90 ports. Predictions are computed with the FES2022b harmonic model, validated against the IHM's official predictions with a median accuracy of ±10 minutes on the open coast. The window finder shows you the next spring days at a glance — or the next neaps, if calm is what you're after — the Apple Watch puts the coefficient right on your watch face, and the Live Activity keeps it on your lock screen while the tide rises or falls.

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