Ocean swells move far faster than the breaking waves we see from the beach. Ever wondered at what speed they travel? Here’s a simple method for calculating it.
The wave generation process in the open ocean is relatively. You probably know how it all begins.
Wind creates waves within a storm zone, also known as fetch. Once the waves escape that windy birthplace, they spread outward like ripples from a stone thrown into a pond.
As the growing circle expands, the energy spreads thinner, so the swell slowly loses size.
During the first day of travel, energy loss is steep. When the distance from the storm doubles, the energy drops by roughly 15 to 20 percent.
After that initial fade, the swell can travel for thousands of miles while still carrying plenty of power. Meteorologists call this process swell decay.
This early energy loss is why the biggest waves usually form close to the storm that created them.
Travel long enough, though, and the swell becomes cleaner and more organized. But how fast does swell exactly travel? What is the speed of an ocean swell?
The good news is that you don’t need advanced physics to understand how fast they go or why some groundswells hit your local break with power and punch.
With a few simple ideas and one small equation, you can estimate swell speed anywhere on Earth.
Why Long-Period Waves Race Ahead
Water inside a traveling wave moves in circular orbits. Those orbits get larger when the wave has more energy and a longer period.
Bigger orbits make the energy slide forward faster. The result is simple: long-period waves outrun short-period waves.
This effect is called radial dispersion. It’s why new groundswells first appear on a forecast chart as sudden jumps in period.
If your home break sits too close to the storm, the short-period leftovers arrive mixed with the clean stuff, giving you messy peaks.
Farther away, the fast, long-period lines sort themselves out and create the organized sets surfers love.
That balance between decay and dispersion creates what many forecasters call a sweet spot: far enough for clean sets, close enough to still pack a punch.
Sets and Interference
A group of waves traveling together at similar speeds forms a wave train.
Inside that train, different wavelengths overlap. Sometimes they combine to form a bigger crest. Sometimes they cancel each other out.
These two behaviors are known as constructive and destructive interference.
The interactions between those overlapping wavelengths shape the sets surfers observe from the beach and get in the lineup.
When the wavelengths are similar, the overlap stretches out and produces many-wave sets with long pauses. When they’re different, the sets are shorter and the lulls are brief.
As the swell travels and the long-period waves pull ahead, the rhythm of the sets gradually shifts.
Inside a wave group, the individual surface waves don’t live very long. One fades, another forms near the back, and the cycle continues while the whole group glides forward.
The Simple Equation Every Surfer Should Know
Forecasters have a handy shortcut for estimating how fast a swell travels in deep water. And we can use it, too.
The speed of the wave group – the speed you may use for predicting arrival times – is approximately:
Speed in meters per second = 0.78 × period in seconds
You don’t need metric units to understand this. Most surfers stick with a simpler ratio:
Group speed ≈ 1.5 × period
That gives the speed in nautical miles per hour, which is basically knots.
So:
- A 10-second swell travels about 15 knots (17.2 miles per hour or 27.7 kilometers per hour);
- A 15-second swell travels about 22 to 23 knots (25.9 miles per hour or 41.7 kilometers per hour);
- A 20-second swell travels about 30 knots (34.5 miles per hour or 55.6 kilometers per hour);
Those numbers match real-world buoy observations.
A buoy 400 miles offshore will see a 20-second swell roughly 13 hours before it hits the coastline, while a 12-second swell might need 20-plus hours to cover the same distance.
But Individual Waves Move Even Faster
Inside that traveling wave group, each individual wave moves at almost double the group’s speed. A common rule of thumb is:
Individual wave speed ≈ 3 × period
A 20-second wave can race along at around 60 knots as it cycles from the back of the set to the front, slows, drops back, and repeats.
Forecasters many times describe it like a conveyor belt inside a moving truck. The group represents the truck. The individual waves are the belt moving on top of it.
The two-speed system confuses many new surfers, which is why most forecast models only list the group speed.
That’s the speed that determines when the swell arrives at your beach. It’s also the speed of the energy, not the water itself.
Do Faster Sells Make Bigger Waves?
Not directly.
Two swells with the same period can break very differently, even if both traveled at the same deep-water speed.
What matters most when the swell hits shallow water is how much energy remains in the wave and how the seafloor shapes that energy.
Bathymetry, refraction, local winds, and angle of approach often play a bigger role than deep-ocean travel speed.
That said, long-period waves usually start with more energy because they formed from stronger winds, longer fetches, or both.
They also lose energy more slowly during long-distance travel. That’s why a 16-second swell frequently feels heavier than a 10-second swell, even if the deep-water heights look similar.
How to Estimate Swell Arrival
You can track an approaching swell yourself using just period and distance. Here’s what you can do:
- Find the offshore buoy farthest from shore but still in line with the swell direction;
- Check the swell period at that buoy;
- Multiply the period by 1.5 to get the travel speed in knots;
- Divide the distance between buoys by that speed;
Example for a 400-mile distance:
30-second swell:
Speed ≈ 30 knots
Time ≈ 400 ÷ 30 ≈ 13 hours
12-second swell:
Speed ≈ 18 knots
Time ≈ 400 ÷ 18 ≈ 22 hours
With a bit of practice, you’ll notice you can generally match the forecast models at their own game.
A final note on what happens when the seafloor rises
Deep-water swells act like smooth stacks of rotating energy cylinders that may stretch hundreds of meters downward.
As long as the seafloor sits out of reach, the cylinders pass through the water without touching bottom.
Once the depth becomes shallow enough, the lower part of the energy hits the seafloor, slows down, and forces the upper part to rise.
That lifting is what makes the wave stand up, pitch, and break.
No matter how fast the swell was moving in the open ocean, the last few hundred meters before the beach depend almost entirely on the shape of the bottom.
So, the quality of your local surf break’s bathymetry determines pretty much the quality of the waves.
Unlike beach breaks, reef breaks, with their stable ocean floor design, are often more consistent and “know” which swell speed, period, height, and direction they “prefer” to deliver pristine surfing conditions.
Words by Luís MP | Founder of SurferToday.com
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