Tide coefficient calculator
The coefficient tells you how big a tide is, from 20 (the smallest) to 120 (the biggest). Get it for any place and day, or type the range of a tide and we give you its coefficient with the Brest formula or with your place’s mean range.
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- ✓ By place or by range
- ✓ Scale from 20 to 120
Work out the coefficient
Chosen place: Charleston · South Carolina, United States
- High tide at 10:05 am (6.8 ft)83
- High tide at 10:17 pm (5.6 ft)82
- Range of the day: 6.8 ft
Coefficient for every day of October 2026
Station: Charleston (Customhouse Wharf).
How to use it
- By place: search for your town, beach or harbor and pick the day. You get the coefficient of each high tide and of every day of the month.
- By range: type the difference between high and low tide and choose the reference: Brest (the official French and Spanish scale) or your place’s mean range.
- Compare it with the scale: 70 is an average tide; 90 and above, spring tides; below 45, neap tides.
The coefficient scale, from 20 to 120
| Coefficient | Tide | What to expect |
|---|---|---|
| 20–44 | Neap tides | The tide barely rises and falls. Weak currents. A poor day for clamming. |
| 45–69 | Low coefficient | Tides below average. |
| 70–89 | Average coefficient | 70 is the average tide for the place. |
| 90–99 | Spring tides | High highs and low lows. Strong currents. Good low tides for tide-pooling and clamming. |
| 100–120 | Big spring tides | Mostly near the equinoxes, with a new or full moon, and even more so with the moon close to Earth (perigee). Watch out for high water on promenades and parking lots. |
- 20the smallest possible
- 45average neap tide
- 70average tide
- 95average spring tide
- 100average equinox spring tide
- 120the biggest possible
The formulas
The official coefficient (Brest)
The coefficient was defined by Laplace and is published by SHOM, the French hydrographic office, for the port of Brest; Spain uses the same one for its Atlantic coast. It compares the range of the tide in Brest with that of an average equinox spring tide, which scores 100.
C = 100 × range / (2 × U) with U = 3.05 m
Range: high water minus low water in Brest, in meters. U is Brest’s “height unit”: half the mean range of equinox spring tides (6.10 m, 20 ft).
The coefficient for each place
In Brest a 10 ft tide is small; in Charleston it is big. So for each place we compute the coefficient from its own tide: the range at that moment compared with the place’s mean range, so that 70 is always the average tide.
On diurnal coasts such as the Gulf of Mexico, the tide follows the moon’s height in the sky rather than its phases. There the exponent k is below 1, so the top 1% of tides lands at 120.
C = 70 × (range / mean range)ᵏ
Mean range: that of the place, computed from its constants. k = 1 on semidiurnal coasts.
Charleston, October 2026
In Charleston the mean range is 5.1 ft. This month the highest coefficient is 85, on October 26, and the lowest 47, on October 18.
The coefficient around the world
- France
- SHOM publishes a single coefficient, Brest’s, for the whole Atlantic and Channel coast. It is part of coastal life: “grandes marées” make the news.
- Spain
- Puertos del Estado and the tide almanacs give the Brest coefficient. It is widely used in Galicia, the Bay of Biscay and the Gulf of Cádiz for shellfishing.
- United States and United Kingdom
- No coefficient is used: people talk about spring and neap tides and the tidal range in feet or meters. Our per-place coefficient works just as well to compare days.
- Mediterranean
- With tides of a few inches, the coefficient changes like anywhere else, but wind and air pressure move the water much more than the tide.
Frequently asked questions
What is a high tide coefficient?
90 and above: those are spring tides, with higher highs and lower lows than usual. They happen twice a month, a day or two after the new and the full moon.
What coefficient is good for clamming or tide-pooling?
From 90 up, low tide uncovers much more of the shore. Coefficients from 100 to 120, near the equinoxes, are the best and also the most dangerous: the tide comes back fast.
When do coefficients of 110 to 120 happen?
When a new or full moon, the moon close to Earth (perigee) and dates near the March and September equinoxes come together. It happens a few times a year.
Why is your coefficient different from the official one?
The official one is Brest’s and applies to a whole coast. Ours is computed from each place’s tide, so it can differ by a few points and peak a day or two earlier or later.
What is the tide coefficient formula?
The official one is C = 100 × range / 6.10, with Brest’s range in meters. For each place we use C = 70 × range / the place’s mean range.
More tools
- Tide height at a given timeHow high the water will be at the time you pick, whether it is rising or falling and how long until high tide.
- Moon calendarEvery moon phase of the year, eclipses, supermoons and the best fishing days. Printable too.
- Solunar calendarMajor and minor periods and the activity rating for every day of the month.
- Nautical unit converterKnots, Beaufort, mph, fathoms, nautical miles, inHg and °F, with tables.
All of this, already worked out for your spot
Every coastal town, beach and harbor has its own page with today’s tide, the yearly tide table, sun, moon and the best time to fish.
Where the data comes from
Our own per-place coefficient, computed from the harmonic constants of its station (NOAA, TICON-4 or the country’s official service) or, where no station is nearby, from those of the global FES2022 and EOT20 models. The Brest formula is SHOM’s (height unit U = 3.05 m).
Spotted a mistake? Tell us · How we calculate tides, sun and moon