Swell period explained: why seconds matter as much as feet

What is swell period?

Period is measured at a fixed point, such as a buoy: the time it takes two crests in a row to pass it. Forecasts list it next to height and direction: “3 ft at 12 s from the SSW” means waves with a significant height of 3 feet, 12 seconds from crest to crest, arriving from the south-southwest.

Period isn’t an isolated number. In deep water, it also sets how long the wave is and how fast it moves. The distance between crests is about 5.12 × T² feet, and each wave travels at about 3.5 × T mph, where T is the period in seconds:

Period	Distance between crests	Speed of each wave
6 s	184 ft (56 m)	21 mph
10 s	512 ft (156 m)	35 mph
14 s	1,000 ft (306 m)	49 mph
18 s	1,660 ft (505 m)	63 mph

The energy, though, travels in groups of waves that move at half that speed. A 14-second swell from a storm 1,250 miles (2,000 km) away takes a little over two days to arrive.

Because long waves move faster, they pull ahead of short ones. So when a new swell shows up, the first waves to arrive have the longest periods, and the period drops as the rest of the swell fills in.

Short and long periods: what each one brings

Wind makes waves, and they get bigger and longer the harder it blows, the longer it lasts and the more open water it crosses. While the wind is still pushing them, they’re wind swell: short, choppy waves of mixed sizes. Once they leave the area where the wind blows, they sort themselves by period and travel as groundswell, in regular lines. Short waves lose energy faster, so far from the storm only the long ones are left.

The National Weather Service (NWS) office in Melbourne, Florida, uses these ranges: local wind waves usually run 3 to 6 seconds; fresh swell from a nearby storm, 7 to 10; and pure swell from far away, more than 10. Here’s what each period usually brings:

Period	What it usually is	What it does at the coast
Under 6 s	Local wind swell	Short, choppy waves that break all over the place. Poor surf and rough going in a small boat.
6 to 9 s	Developed wind swell or nearby swell	A bit more shape. Weak to fair surf, better with offshore wind.
10 to 13 s	Groundswell	Clean lines and clear sets. Breaks a bit bigger than the height suggests.
14 to 17 s	Groundswell from distant storms	Lots of energy. Sets far apart with long lulls; breaks noticeably bigger and runs farther up beaches and rocks.
18 s or more	The leading edge of a very distant swell	Can carry little height but a lot of power, and arrives ahead of the rest of the swell. Be careful on rocks and jetties.

Why longer-period swell carries more energy

Wave theory sums it up in two steps. First, the energy of a wave over each square meter of ocean is E = ρ · g · H² / 8, where ρ is the density of water, g is gravity and H is the wave height. Energy grows with the square of the height: double the height and you get four times the energy.

Second, that energy travels at the speed of the wave groups, which in deep water grows in proportion to the period. Multiply the two and the power arriving along each meter of wave crest, in deep water, comes to about 0.5 × Hs² × T kilowatts, with Hs the significant wave height in meters and T the period in seconds (strictly, the “energy period,” a little shorter than the dominant period):

Swell	Power per meter of wave crest
3 ft (0.9 m) at 6 s	about 2.5 kW
3 ft at 12 s	about 5 kW
6 ft (1.8 m) at 12 s	about 20 kW
10 ft (3 m) at 15 s	about 68 kW

At the same height, twice the period means twice the power. And you see it when the waves break. As a wave approaches the coast, it starts to feel the bottom once the depth drops to about half its length: about 160 feet (50 m) at 8 seconds and about 650 feet (200 m) at 16. From there it slows, shortens and grows taller. Long waves feel the bottom sooner and grow more.

Paul Komar and Michael Gaughan of Oregon State University studied breaker height with laboratory and field data and summed it up in a formula built on the idea that the flow of wave energy is conserved: Hb = 0.39 · g^0.2 · (T · H0²)^0.4, where Hb is the height of the wave as it breaks and H0 is its height offshore (in metric units). Starting from 3 feet offshore, it gives a breaker of about 4.3 feet at 8 seconds and about 5.7 feet at 16. TidePlanet uses this formula, cut by 15% for the energy lost over the bottom, for the surf at the shore on its Surf tab.

The Surf tab also shows the energy of the swell in kilojoules per meter of wave crest. It’s the energy in one wavelength, ρ · g² · H² · T² / 16π, so the period counts squared. An 8-foot (2.5 m) swell at 10 seconds comes to about 1,200 kJ; the same height at 14 seconds, about 2,400.

Typical swell periods on U.S. coasts

NOAA’s National Data Buoy Center (NDBC) publishes the dominant period that each of its buoys records. We went through the records for 2023 and 2024 from five of them:

Coast (NOAA buoy)	Typical summer period (June to August)	Typical winter period (December to February)	Winter time at 12 s or more
Outer Banks, N.C. (41025, Diamond Shoals)	7 s	8 s	9%
New Jersey and Long Island (44025)	7 s	7 s	4%
San Francisco (46026)	8 s	14 s	75%
Southern California (46222, San Pedro)	12.5 s	13 s	69%
North Shore of Oahu (51201, Waimea Bay)	8 s	13 s	68%

The “typical” period is the median: half the time it was shorter, half the time longer.

• East Coast. Off Diamond Shoals, the dominant period was under 10 seconds 73% of the time, and 89% of the time in summer. In the fall, short-period waves often build behind cold fronts, while tropical systems send in longer-period swell at the same time, as the NWS office in Wilmington, North Carolina, explains. Compare the waves at Nags Head.
• West Coast. Off San Francisco, winter periods ran 12 seconds or more three quarters of the time, and 16 seconds or more about one hour in five. Off Southern California, long periods show up in summer too: at San Pedro, 12 seconds or more about half the time from June to August. See the waves at Huntington Beach.
• Hawaii. Researchers at the University of Hawaii describe four regimes: trade-wind waves from the northeast with a typical peak period of about 9 seconds; North Pacific swell in winter, with the biggest yearly events peaking at 14 to 18 seconds; south swell in summer, at 14 to 22 seconds; and Kona storm waves at 8 to 11 seconds. Check the waves in Haleiwa on Oahu’s North Shore.

Dominant period vs. average period

Not every forecast or buoy gives the same period, so it helps to know which one you’re looking at:

• Dominant (or peak) period: the period of the waves carrying the most energy. NDBC lists it as DPD. It’s the best guide to how a swell will break.
• Average period: the average of all the waves (APD at NDBC). Mix in some small wind waves and it drops, even when the groundswell is still long. The NWS office in Melbourne goes as far as saying to ignore it, because no real wave matches it.

When two wave trains overlap, say a 14-second groundswell and 5-second wind chop, the dominant period can jump from one to the other from one hour to the next, depending on which carries more energy. That’s why it’s more useful to see them separately.

On TidePlanet, each place’s Waves tab gives the average period of the whole sea plus, separately, the height and period of the swell. The Surf tab splits the swell and the wind swell, each with its own height, period and direction. For the full picture, see how to read a surf forecast.

What it means for surfing, fishing and the beach

Surfing

For surfing, period tells you as much as height. In TidePlanet’s surf score, wind swell under 6 seconds costs 2.5 points; at 10 seconds, period neither adds nor takes away; and a swell of 17 seconds or more adds 2. With long periods, sets come farther apart and you wait longer between waves, but each one hits harder. At 15 seconds or more with some size, waves break with a lot of power: not a day for learning.

Fishing from rocks and jetties

This is where long-period swell is most treacherous. It arrives in sets: plenty of ordinary waves, then suddenly one that runs up the rocks or over the jetty. The NWS office in Medford, Oregon, explains that water washing over jetties and rocks often comes with long-period swell of more than 12 seconds and/or a rising tide, and that a small rise in the tide can mean much more water coming over.

• If the forecast calls for swell longer than 12 seconds, stay off rocks and jetties exposed to open ocean.
• Watch the water for a good while before you climb down, not just a few seconds: sets can take a long time to show up.
• Never turn your back on the ocean, and always keep a way out uphill.
• Remember that on a rising tide, each wave reaches a little higher than the last.

Beachgoers

The same thing happens on the sand. Sneaker waves run much farther up the beach than the others, without warning. According to a 2018 newsletter from the NWS office in Medford, they had killed more than 20 people along the Oregon coast since 2000, nearly all between October and April, and most came with swells longer than 12 seconds. According to the NWS, ordinary waves can roll in for 30 minutes or more before a sneaker wave hits, which lulls people into a false sense of security.

If the forecast calls for long-period swell, stay well back from the water, especially on steep beaches and at high tide, and keep an eye on children. For currents in inlets and river mouths, see our guide to tidal currents, and before walking out on flats and rocks, read up on tide safety.