Swell Travel Time Calculator

How Swell Travels

Ocean swell is generated by storms and travels as groups of waves across open water.
The speed at which these wave groups travel is called the group velocity, and for deep-water waves it is half the speed of the individual waves inside the group.

Group speed (km/h) = 1.56 x period (seconds) x 3.6 / 2

Simplified: swell group speed (km/h) = approximately 2.81 x period in seconds.

A 12-second swell travels at roughly 33.7 km/h (18.2 knots). A 16-second swell travels at about 45.0 km/h (24.3 knots). Long-period swell does not just feel better in the water, it gets to you sooner from the same storm.

Group speed, not wave speed - this trips people up

Watch an individual crest inside a swell train and it moves at twice the speed of the train itself. It rises at the back of the group, rolls forward through it, and disappears off the front. The energy, and therefore the surf, travels at the slower group speed.

So there are two correct answers to “how fast is a 12-second swell”. The individual waves move at 67.4 km/h, which is what the site’s wave period and speed page reports as phase speed. The group, which is the thing that actually arrives at your beach, moves at 33.7 km/h. Forecasting arrival always uses the group speed, which is what this page does. Use the wrong one and you will be sitting on the sand half a day early.

Why Period Matters for Forecasting

Two swells from the same storm but at different periods arrive at different times. The longer-period energy travels faster and gets there first, even though it left at the same moment. This is called period sorting, and it is why forecasts show the period dropping steadily through a swell event: the fastest, cleanest energy shows up first and the short-period slop follows behind it.

Storm Distance Estimation

Surf forecasters usually express swell origin in nautical miles from the forecast point. 1 nautical mile = 1.852 km. A North Pacific storm 2,500 nm off a California beach sends swell 4,630 km to reach the shore, which at 14 seconds is a shade under five days.

What This Does Not Account For

Swell loses energy to seafloor friction as it enters shallow water (depths less than half the wavelength). Deep-water group velocity holds until the swell reaches the continental shelf, so for an ocean crossing the shallow leg is a rounding error on the total. Currents and the exact storm position matter more. Treat the answer as accurate to a few hours over a multi-day crossing, not to the minute.