Swell Travel Time Calculator
Estimate when a swell will arrive at your surf spot.
Enter the swell period, storm distance, and direction to forecast arrival time and wave speed.
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, which for deep-water waves is half the speed of individual waves within the group.
Deep-water group velocity (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 44.9 km/h (24.3 knots). Longer period swells not only feel better in the water but arrive faster from distant storms.
Why Period Matters for Forecasting
Two swells from the same storm but at different periods will arrive at your spot at different times. The longer-period energy leaves first (it travels faster) and arrives first, even though it was generated at the same time as shorter-period waves. This is called period sorting, and it is why swell forecasts often show a gradual change in period leading up to peak swell.
Storm Distance Estimation
Surf forecasters typically express swell origin in nautical miles from the forecast point. Converting: 1 nautical mile = 1.852 km. A North Pacific storm 2,500 nm from a California beach generates swell traveling 4,630 km to reach the shore.
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 applies until the swell hits the continental shelf. For most ocean crossings this is a minor factor.