What wave height is sea state 4?

Sea state 4 (moderate) is a significant wave height of 1.25–2.5 m (4.1–8.2 ft). State 3 (slight) is 0.5–1.25 m, state 5 (rough) 2.5–4 m and state 6 (very rough) 4–6 m. Individual waves can be about twice the significant height.

What is the Douglas sea scale?

A scale devised by Captain H. P. Douglas of the Royal Navy in 1921 with two parts: a wind-sea scale graded by wave height and a swell scale graded by wave height and length. The WMO adopted the wind-sea part, with heights from 0 to over 14 m, as its sea state code 3700.

How do Beaufort force and sea state relate?

Only loosely. The Beaufort wave heights are for a well-developed sea in the open ocean. Force 5 (17–21 kn) gives a probable 2 m (sea state 4, moderate) and force 7 gives 4 m (state 5–6). Near the coast, in short fetch or soon after the wind rises, waves are lower; in a swell from a distant storm they can be higher with little wind.

What is significant wave height?

The average height (trough to crest) of the highest one-third of the waves, written Hs or H1/3. It is close to what an experienced observer reports. In a given sea about 1 wave in 1,000 reaches roughly 1.9 times Hs, which is why the probable maximum is quoted separately.

By height and wavelength. Height is low (under 2 m), moderate (2–4 m) or high (over 4 m); length is short (under 100 m), average (100–200 m) or long (over 200 m). A 2 m swell at a 12 s period is about 225 m long, so it is moderate and long: swell code 5 (moderate rough).

What sea state is dangerous for small boats?

There is no single cutoff. It depends on the boat, the crew, wave period and steepness, wind against tide and local depth. Short, steep seas of state 4 can be harder for a small boat than a long state 5 swell. Check the official marine forecast and the boat's design category.

Sea State Reference

Sea state 0–9 is set by significant wave height: 3 = slight (0.5–1.25 m), 4 = moderate (1.25–2.5 m), 5 = rough (2.5–4 m), 6 = very rough (4–6 m). Enter a wave height, wind speed or swell period to get the code.

Wave heights: m ft

Sea state from significant wave height

Enter a wave height (Hs).

WMO sea state code 3700 (Douglas wind-sea scale)
Code	Description	Wave height (Hs)	Sea surface
0	Calm (glassy)	No waves	No waves. The surface is mirror-flat, although swell from a distant system may still be running.
1	Calm (rippled)	0–0.1 m	Ripples with the look of scales, no foam crests.
2	Smooth (wavelets)	0.1–0.5 m	Small wavelets; crests look glassy and do not break.
3	Slight	0.5–1.25 m	Large wavelets; crests begin to break and scattered white horses appear.
4	Moderate	1.25–2.5 m	Small waves becoming longer, with fairly frequent white horses.
5	Rough	2.5–4 m	Moderate waves of a more pronounced long form; many white horses and some spray.
6	Very rough	4–6 m	Large waves form; white foam crests are extensive and spray is likely.
7	High	6–9 m	The sea heaps up and foam from breaking waves is blown in streaks along the wind.
8	Very high	9–14 m	Very high waves with long overhanging crests; dense foam streaks; the surface looks white and visibility is affected.
9	Phenomenal	Over 14 m	Exceptionally high waves; the air is filled with foam and spray and visibility is seriously reduced.

Beaufort force from wind speed

Enter a mean wind speed.

Beaufort wind force scale with WMO probable wave heights
Force	Description	Knots	m/s	km/h	mph	Probable wave	Max wave	Sea state
0	Calm	< 1	0–0.2	< 1	< 1	—	—	0
1	Light air	1–3	0.3–1.5	1–5	1–3	0.1 m	0.1 m	1
2	Light breeze	4–6	1.6–3.3	6–11	4–7	0.2 m	0.3 m	2
3	Gentle breeze	7–10	3.4–5.4	12–19	8–12	0.6 m	1 m	3
4	Moderate breeze	11–16	5.5–7.9	20–28	13–18	1 m	1.5 m	3–4
5	Fresh breeze	17–21	8.0–10.7	29–38	19–24	2 m	2.5 m	4
6	Strong breeze	22–27	10.8–13.8	39–49	25–31	3 m	4 m	5
7	Near gale	28–33	13.9–17.1	50–61	32–38	4 m	5.5 m	5–6
8	Gale	34–40	17.2–20.7	62–74	39–46	5.5 m	7.5 m	6–7
9	Strong gale	41–47	20.8–24.4	75–88	47–54	7 m	10 m	7
10	Storm	48–55	24.5–28.4	89–102	55–63	9 m	12.5 m	8
11	Violent storm	56–63	28.5–32.6	103–117	64–72	11.5 m	16 m	8
12	Hurricane force	≥ 64	≥ 32.7	≥ 118	≥ 73	14+ m	—	9

Wave heights are for a well-developed wind sea in the open ocean (WMO, Met Office). In enclosed waters, short fetch or a rising wind they are lower. Speed bands are the WMO 10 m mean wind; the m/s, km/h and mph columns are rounded conversions.

Swell code from height and period

Swell height

Period or wavelength

Enter swell height and period.

Douglas swell scale
Code	Description	Wavelength	Height
0	No swell	—	—
1	Very low	Short or average (< 200 m)	Low (< 2 m)
2	Low	Long (> 200 m)	Low (< 2 m)
3	Light	Short (< 100 m)	Moderate (2–4 m)
4	Moderate	Average (100–200 m)	Moderate (2–4 m)
5	Moderate rough	Long (> 200 m)	Moderate (2–4 m)
6	Rough	Short (< 100 m)	High (> 4 m)
7	High	Average (100–200 m)	High (> 4 m)
8	Very high	Long (> 200 m)	High (> 4 m)
9	Confused	Not definable	Not definable

Period is converted with the deep-water relation L = gT²/2π ≈ 1.56·T² m: 8 s ≈ 100 m, 10 s ≈ 156 m, 12 s ≈ 225 m. In shallow water swell is shorter for the same period.

Summary

The WMO sea state code (code table 3700) is the wind-sea half of the Douglas scale: it grades the sea from 0 (calm, glassy) to 9 (phenomenal) by significant wave height, the mean of the highest third of the waves. The Beaufort scale grades wind force 0–12; its wave heights are the probable and probable-maximum heights of a well-developed sea in the open ocean, so a force gives a typical sea state only after the wind has blown long enough over enough fetch. The Douglas swell scale grades swell 0–9 by height (low under 2 m, moderate 2–4 m, high over 4 m) and wavelength (short under 100 m, average 100–200 m, long over 200 m).

How it works

Douglas tab: enter a significant wave height in metres or feet to get the sea state code. Boundary values belong to the lower code, so exactly 2.5 m is state 4 (moderate).
Beaufort tab: enter wind speed in knots, m/s, km/h or mph. It is rounded to the nearest knot and matched to the WMO knot bands, then shown with the probable and maximum wave height and the typical sea state.
Swell tab: enter swell height and either the swell period or the wavelength. Period is converted with the deep-water relation L = gT²/2π ≈ 1.56·T² m, so 10 s ≈ 156 m (average length).
Filter rows by text or by a value such as 3 m, 10 ft or 25 kn, switch every wave height between metres and feet, and click a row for the full description.

Use cases

Decode the sea state or "wave height 1.5 to 2.5 m" in a coastal or offshore forecast.
Turn a buoy reading of significant wave height into the code used in a ship's log or weather report.
Estimate the sea that a forecast wind force will build once the sea is fully developed.
Classify a 2 m, 12-second groundswell for a passage plan or surf report.
Revise Douglas, Beaufort and swell terms for RYA, USCG or STCW exams.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu

Sea State Reference

Use the Sea State Reference

Summary

How it works

Use cases

Frequently Asked Questions