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Understanding swell: height, period and direction
Written by the Isobath editors · Published 2026-10-16
A surf forecast comes down to three numbers: a height, a period and a direction. Reading them together changes everything, because two swells of the same height can make for completely different sessions.
Swell and wind sea are not the same thing
Wind blowing over the ocean first creates wind sea: short, messy waves that pile on top of each other. Once they leave the storm area they stretch out, sort themselves into regular trains and become swell, which can cross an entire ocean basin. The swell reaching the French Atlantic coast usually comes from low-pressure systems that formed several days earlier, far out to sea.
A good forecast separates the two. A 1.5 m wind sea is chop; a 1.5 m swell means clean sets that rise and break in an orderly way. In the glossary, these are called windsea and swell.
Height: a number offshore, not at the break
The height a model gives is the swell height in deep water. Near the shore the seabed rises, the wave slows, steepens and grows, then breaks once the water is too shallow to carry it. The result depends on the spot's seabed: the same swell gives 1 m over a wide, gentle sandbar and much more over a reef that shoals abruptly.
That is why Isobath gives a breaking size, spot by spot, as a range rather than a single figure. Model uncertainty is real: the gap between two models tells you how cautious to be.
Period: the number that sets the power
The period is the time between two successive crests, in seconds. The longer it is, the more energy the wave carries and the farther it has travelled. As a first approximation, power grows with the square of the height and in proportion to the period: a 1.5 m swell at 14 s carries far more energy than a 1.5 m sea at 6 s.
Rules of thumb: below 8 s the sea is short and often choppy. Between 9 and 12 s you have a well-formed swell, the most pleasant for most sandy beaches. Beyond 14 s, waves are long, fast and powerful, with sets that can catch you out; this is what the long-period warning on spot pages is about. See period and peak period.
Direction: where the swell comes from, where the beach faces
A swell direction is always the direction it comes from. A west-north-west swell travels from the open sea towards the east-south-east. Each spot has an orientation: the direction in which it looks out to sea. The more head-on the swell, the more force it arrives with; the more oblique, the more size it loses and the more it spreads along the shore.
Headlands, islands, jetties and the seabed can also screen part of the horizon: a north-west-facing spot may receive nothing from a south-westerly swell. On each spot page the sentence "works with swell from… to…" gives that spot's open window.
Reading the three numbers together
An example: 2 m, 13 s, from the west. On a west-facing beach break on the Landes coast, that is an organised swell hitting head-on, and it will produce waves much bigger than "2 m" suggests once you are in the water. By contrast, 2 m, 6 s from the north-west on the same spot is a short, angled, smaller and messier sea.
Then add two filters. Wind first: an offshore wind smooths the face of the waves, an onshore wind wrecks it; see the spots by wind direction. Tide next: a sandbank does not behave the same at low and high tide, and most spot pages state the favourable range.
And if you are just starting out?
Aim for small waves, a medium period (8 to 11 s) and light or offshore wind. A long swell, even a modest one, demands experience: more power, more currents. The gentle waves today and spots for beginners lists apply exactly that filter, and the safety page covers the hazards you should never ignore.