Sea state converter
Convert a sea state from a marine forecast to wave height, or a wave height to its name on the Douglas scale.
That's 1.25× as rough as a surfing swell, while a North Sea storm is 2.8× rougher. That is sea state 5: rough.
What it means
How rough is that sea?
Significant wave height: the average of the highest third of waves. Single waves can be twice as high.
Every unit
That sea in every unit
Learn
Three things worth knowing
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Moderate is under 2.5 m
Slight 0.5–1.25 m, moderate 1.25–2.5, rough 2.5–4, very rough 4–6, high 6–9 m.
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Admiral Douglas
Captain Henry Percy Douglas of the Royal Navy devised the scale in 1917 for hydrographic surveys; the WMO adopted its wave part.
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Not the biggest wave
Forecast heights are averages. One wave in a few thousand reaches about twice the significant height.
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Popular sea conversions
Background
About the Douglas sea scale
Marine forecasts describe the sea by its significant wave height, the average of the highest third of waves. The Douglas sea scale (WMO code 3700) names it: 3 slight (0.5–1.25 m), 4 moderate (1.25–2.5 m), 5 rough (2.5–4 m), up to 9 phenomenal (over 14 m).