A tide coefficient is a number, typically ranging from 20 to 120, that tells you how dramatic a given tide will be compared to an average tide. A coefficient of 100 or higher means an exceptionally large tidal range, where the water rises very high and drops very low. A coefficient near 20 means the opposite: the sea barely moves between high and low tide. The system is most widely used along the Atlantic coast of France, though it applies anywhere tides follow a similar pattern.
How the Scale Works
The tide coefficient is calculated relative to a reference tide at a specific location (the port of Brest, in France, serves as the traditional baseline). A coefficient of 70 represents a roughly average tide. Below 70, tides are considered small. Above 70, they’re considered large. The scale bottoms out around 20 and peaks near 120, though values above 118 or so are rare and represent the most extreme tides possible under normal astronomical conditions.
What makes the number useful is its simplicity. Rather than checking tide tables for exact water heights at a specific port, you can glance at a single number and immediately know whether the day will bring big tidal swings or gentle ones. A coefficient of 45 means a calm, small tide. A coefficient of 95 means a big one. A coefficient of 110 or above is exceptional.
What Pushes the Coefficient Up or Down
The coefficient is driven almost entirely by the positions of the sun, moon, and Earth relative to each other. Three factors matter most.
The first is the moon’s phase. During new moons and full moons, the sun and moon are aligned with the Earth, and their gravitational pulls reinforce each other. This produces spring tides, which have larger-than-average ranges and higher coefficients. During first-quarter and third-quarter moons, the sun and moon pull at right angles to each other, partially canceling out their effects. These are neap tides, with smaller ranges and lower coefficients. This cycle repeats roughly every two weeks.
The second factor is the moon’s distance from Earth. The moon’s orbit is not a perfect circle; once a month it swings closer (perigee) and once a month it drifts farther away (apogee). When the moon is at perigee, its gravitational pull is stronger, and tidal ranges increase. When it’s at apogee, tidal ranges shrink.
The third factor is the Earth’s distance from the sun. Earth is closest to the sun around January 2 each year, which slightly enhances tidal ranges. It’s farthest around July 2, which slightly reduces them. This effect is smaller than the lunar factors but still contributes.
The highest coefficients occur when all three factors align: a new or full moon, the moon at its closest approach, and the Earth near its closest point to the sun. When these conditions converge, coefficients can climb above 115, producing the largest tidal swings of the year. Conversely, the lowest coefficients happen when the moon is in a quarter phase, far from Earth, and the Earth is far from the sun.
Why It Matters for Fishing and Foraging
Shore fishers and shellfish foragers pay close attention to tide coefficients because a high coefficient means the sea retreats much farther than usual at low tide. Rock pools, sand flats, and reef areas that are normally underwater become accessible, sometimes exposing stretches of coastline you can’t reach during average tides. For this reason, the best shore fishing and foraging days tend to coincide with coefficients above 90 or 100.
The tradeoff is that the water also comes back in farther and faster. A large tidal range means stronger currents during the flood (incoming) and ebb (outgoing) phases. If you’re wading out on an exposed flat during a high-coefficient low tide, you need to time your return carefully, because the rising water can cover ground quickly and cut off escape routes.
Safety and Coastal Flooding
High coefficients combined with certain weather conditions can create dangerous situations. A coefficient of 100 or more paired with onshore winds or a storm can push water levels well beyond normal high-tide marks, flooding coastal roads, sea walls, and low-lying areas. The coefficient alone doesn’t cause flooding, but it sets the stage. Wind, atmospheric pressure, and wave action then determine whether the tide stays within bounds or overtops defenses.
Coastal authorities in France routinely reference the coefficient when issuing warnings. If a storm is forecast to arrive during a period of high coefficients, the risk of coastal flooding rises significantly compared to the same storm arriving during neap tides with a coefficient of 40 or 50.
How to Read a Tide Coefficient Forecast
Tide coefficients are published in advance because they’re based on predictable astronomical cycles. French tide tables, marine weather services, and many fishing websites list the coefficient for each day alongside standard tide times and heights. Some quick reference points:
- 20 to 45: Very small tides. Minimal difference between high and low water. Weak currents.
- 45 to 70: Below-average tides. Nothing remarkable in either direction.
- 70 to 90: Above-average tides. Noticeably more shoreline exposed at low tide, stronger currents.
- 90 to 100: Large tides. Good conditions for shore fishing and foraging, but requires awareness of fast-moving water.
- 100 to 120: Exceptional tides. Maximum shoreline exposure, strongest currents, and the highest risk of coastal flooding if weather conditions are unfavorable.
The coefficient changes gradually from day to day, building over about a week from neap to spring tides and then declining again. You’ll typically see two periods of high coefficients and two periods of low coefficients each month, tracking the lunar cycle. Planning a beach outing or fishing trip a week or two in advance is straightforward because the astronomical patterns that drive coefficients are predictable years ahead of time.