What is the Douglas Sea Scale?

The Douglas Sea Scale is a standardized system developed by the British Meteorologist H. P. Douglas in the 1920s. It classifies the state of the sea on a scale from 0 (glassy calm) to 9 (phenomenal), based on the significant wave height — the average of the highest one-third of waves.

How does the Beaufort Scale differ from the Douglas Scale?

The Beaufort Scale (0–12) describes wind force and its effects on both the sea surface and land objects. The Douglas Scale (0–9) focuses specifically on wave height and sea surface appearance. A Beaufort reading can help predict the eventual sea state, but wave heights also depend on fetch, water depth, and swell from distant storms.

What is significant wave height?

Significant wave height (SWH or H₁/₃) is the average height of the highest one-third of waves observed over a period. It closely matches the wave height an experienced observer would report visually and is the standard metric used in marine forecasts and sea state codes.

What sea state is considered dangerous for small vessels?

There is no universal safe/unsafe sea-state cutoff for every small vessel. Risk depends on vessel type and size, seaworthiness, operator experience, wave period and steepness, wind, current, and local conditions. Small Craft Advisory criteria also vary geographically. Check the official local marine forecast and vessel limits before departure.

How are sea states used in marine weather forecasts?

Forecast offices issue sea state codes in GRIB2 model output, NAVTEX bulletins, and Coastal/Offshore Zone forecasts. Mariners use these codes to estimate expected wave heights and plan safe passages, set crew watches, and decide whether to seek shelter.

Sea State Reference

Look up Douglas states 0–9 from significant wave height or Beaufort forces 0–12 from wind speed, with metric and imperial wave values.

Douglas state from significant wave height

Enter a non-negative value.

Wave height: m ft

State	Description	Wave Height	Typical Conditions

Force	Description	Wind Speed	Wave Height	Sea Conditions

Summary

Look up Douglas states 0–9 from significant wave height or Beaufort forces 0–12 from wind speed, with metric and imperial wave values.

How it works

Select the Douglas or Beaufort tab to switch between scales.
Enter significant wave height on the Douglas tab or wind speed in knots on the Beaufort tab for a reverse lookup.
Use the search box to filter rows by code, description, or a numeric wave value such as 3 m or 10 ft.
Click a row, or focus it and press Enter or Space, to read the full description.
Use the unit toggle to switch wave heights between meters and feet.

Use cases

Interpret GRIB2 and NAVTEX sea state codes in marine weather forecasts.
Brief crew or passengers on expected conditions before a passage.
Cross-reference Douglas and Beaufort codes in a ship's logbook.
Study for maritime certification exams covering sea state terminology.
Evaluate reported sea states when analyzing voyage data or incident reports.
Teach new sailors the relationship between wind force and wave development.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu

Sea State Reference

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Summary

How it works

Use cases

Frequently Asked Questions