Wave Period and Speed Calculator

Wave period — the time in seconds between successive wave crests passing a fixed point — is one of the most important numbers in a surf forecast. A longer period means more energy, faster waves, and generally better surf.

In deep water, wave speed and wavelength are determined entirely by period:

Wave speed (m/s) = g × T / (2π) ≈ 1.56 × T Wavelength (m) = g × T² / (2π) ≈ 1.56 × T²

Where:

• T = wave period in seconds
• g = 9.81 m/s² (gravitational acceleration)

Period guide for surfers:

• Under 8 seconds: Local wind swell. Short, choppy, close together. Weak and difficult to surf.
• 8–10 seconds: Average swell. Reasonable surf for most breaks.
• 10–13 seconds: Good groundswell. Powerful, well-organised waves. Most breaks light up.
• 13–16 seconds: Excellent long-period swell. Strong, long-breaking waves. Big wave spots come alive.
• 16–20+ seconds: Major groundswell. Can travel thousands of kilometres from storms. World-class conditions at appropriate breaks.

Wave energy increases with the square of height and linearly with period — a 2-metre wave at 15 seconds carries roughly four times the energy of a 1-metre wave at the same period.

Shoaling and breaking: In shallow water, waves slow down and their height increases (shoaling). The ratio of wave height to water depth at breaking is approximately 0.78 — a wave breaks when the water depth is about 1.3× the wave height. This is why reefs and sandbars create consistent breaking waves.