Swell Period — What the Seconds Mean
Swell period is the time in seconds between successive wave crests passing a fixed point. It is the best single predictor of surf quality, because period determines how deep a wave's energy reaches and therefore how it behaves in shallow water.
If you only look at one number on a forecast, look at this one. Height tells you how much water is moving; period tells you how much energy is behind it and how organised it is.
Period tells you how deep the wave reaches
A wave's orbital motion extends downward to roughly half its wavelength, and wavelength grows with the square of period. A 6-second wind wave stirs water perhaps 9 metres down. An 18-second groundswell reaches past 120 metres.
That is why long-period swell "feels the bottom" far offshore, begins refracting and shoaling much earlier, and stands up dramatically on a reef that short-period chop rolls straight over.
The practical bands
Under 8 seconds is local wind swell: weak, disorganised, closing out. 8–12 seconds is mixed — usually rideable, often unremarkable. 12–16 seconds is genuine groundswell, generated by a distant storm and arriving in orderly sets. Beyond 16 seconds is long-period swell that will break far bigger than its offshore height suggests, and which shuts down some breaks entirely while switching others on.
Period sorts the swell for you
Long-period energy travels faster than short-period energy from the same storm — a property called dispersion. The longest periods arrive first, which is why the leading edge of a new swell is often cleaner and more powerful than its peak, and why a rising period reading is one of the most reliable signals in forecasting.
See it on live data
The fastest way to make any of this concrete is to go and look at the numbers being described.
Common questions
What is a good swell period for surfing?▾
What does "3ft at 12 seconds" mean?▾
Why does the period sometimes drop suddenly?▾
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