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?

Douglas Sea State 5 (rough, 2.5–4 m / 8–13 ft) begins to pose risks for vessels under about 10 m (33 ft). State 6 and above (very rough to high seas) is hazardous for most recreational craft. Always consult an official marine forecast before departing.

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

Name: Sea State Reference
Availability: InStock
Author: Nham Vu

Look up wave height ranges, wind speeds, and sea descriptions for every Douglas (0–9) and Beaufort (0–12) sea state code.

Wave height: m ft

State	Description	Wave Height	Typical Conditions

Force	Description	Wind Speed	Wave Height	Sea Conditions

Summary

Look up wave height ranges, wind speeds, and sea descriptions for every Douglas (0–9) and Beaufort (0–12) sea state code.

How it works

Select the Douglas or Beaufort tab to switch between scales.
Use the search box to filter rows by code number, description, or wave height.
Click any row to expand it and read the full description of sea conditions.
Hover over the wave height bar to see the numeric range in meters and feet.
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: 2026-06-11 · Reviewed by Nham Vu