What is a tide coefficient?
The tide coefficient is a number between 20 and 120 that measures how big the day’s tide is. The higher it is, the lower the sea goes and the higher it rises: the tidal range — the height difference between low and high water — is large.
The coefficient is the same all along the Channel–Atlantic coast: it is computed relative to the port of Brest. On a given day it is identical in Saint-Malo, Brest or Biarritz. Only the water heights change from place to place.
The scale, 20 to 120
- 20 – 40Neap tidesSmall range: the sea barely moves.
- 45 – 70Average tideModerate range, the most common case.
- 70 – 95Spring tidesLarge range: the sea uncovers the foreshore widely.
- 95 – 120Great tidesMaximum range. 120 is the theoretical maximum, reached a few days a year at the equinoxes.
Why it matters
- Foraging: a high coefficient (95+) uncovers the foreshore very low — the best harvests.
- Currents: the higher the coefficient, the stronger the tidal currents (swimming, boating, kayaking).
- Safety: with a high coefficient the sea comes back fast and far. On the foreshore, watch the incoming tide.
How it is computed
The coefficient depends on the positions of the Moon and the Sun. It peaks at spring tides, at new and full moon, when Moon and Sun are aligned and their pulls add up; it is lowest at neap tides, at the first and last quarters. It was defined by the French SHOM from the tidal range at Brest (height unit ≈ 3.05 m). On coefmaree it is computed from the IFREMER atlas: an estimate, very close to but not strictly identical to the coefficient published by the French SHOM.