What is the Beaufort Scale?

The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea and on land. Francis Beaufort devised it in 1805; it became the standard for Royal Navy log entries in the late 1830s and was accepted internationally at the 1853 Brussels conference. It runs from Force 0 (calm, smoke rises vertically, sea like a mirror) to Force 12 (hurricane force, sea completely white with driving spray).

What wind speed is Force 7 on the Beaufort Scale?

Force 7 (near gale) is 28–33 knots, 13.9–17.1 m/s, 32–38 mph or 50–61 km/h. The probable wave height in the open sea is 4 m (maximum about 5.5 m); the sea heaps up and white foam from breaking waves begins to be blown in streaks. On land whole trees are in motion and walking into the wind is difficult.

What is the Beaufort scale in m/s?

WMO limits in m/s are: 0 = 0–0.2, 1 = 0.3–1.5, 2 = 1.6–3.3, 3 = 3.4–5.4, 4 = 5.5–7.9, 5 = 8.0–10.7, 6 = 10.8–13.8, 7 = 13.9–17.1, 8 = 17.2–20.7, 9 = 20.8–24.4, 10 = 24.5–28.4, 11 = 28.5–32.6 and 12 = 32.7 m/s or more. They are the 10-minute mean wind at 10 m above the surface.

What is the formula for the Beaufort scale?

The empirical relation is v = 0.836 × B^(3/2) m/s, or v = 1.625 × B^(3/2) knots, where v is the 10 m wind speed and B the Beaufort number. Rearranged, B = (v ÷ 0.836)^(2/3). For 12 m/s: (12 ÷ 0.836)^(2/3) = 5.9, which falls in the Force 6 band. The official force is read from the table bands; the formula gives the decimal value.

What wave height goes with each Beaufort force?

The WMO table gives probable (and maximum probable) open-sea wave heights: Force 3 0.6 m (1 m), Force 4 1 m (1.5 m), Force 5 2 m (2.5 m), Force 6 3 m (4 m), Force 7 4 m (5.5 m), Force 8 5.5 m (7.5 m), Force 9 7 m (10 m), Force 10 9 m (12.5 m), Force 11 11.5 m (16 m) and Force 12 14 m or more. They apply to well-developed wind waves far from land; near the coast, in short fetch, or just after the wind rises the sea is lower.

How is Beaufort Force different from wind speed in knots?

Knots measure the actual wind speed (nautical miles per hour), while a Beaufort force is a category covering a range of speeds. Force 6 covers 22–27 knots, for example. Marine forecasts use forces because each one corresponds to recognizable sea conditions that can be judged without instruments.

What Beaufort force is a gale?

A gale is Force 8 (34–40 knots). Force 9 is a strong gale (41–47 kn; the UK Met Office says "severe gale"), Force 10 a storm (48–55 kn), Force 11 a violent storm (56–63 kn) and Force 12 hurricane force (64 kn or more). Force 7 (28–33 kn) is a near gale.

Does the Beaufort Scale go above 12?

The WMO scale ends at Force 12 (64 knots or more). Forces 13–17 were added in 1946 for tropical cyclones and are still used in Taiwan, mainland China and Vietnam (Force 17 is above 108 knots), but the WMO Manual on Marine Meteorological Services defines the scale only up to 12.

How do I convert km/h to Beaufort Force?

Round the speed to a whole km/h and find its band: 1–5 km/h is Force 1, 6–11 Force 2, 12–19 Force 3, 20–28 Force 4, 29–38 Force 5, 39–49 Force 6, 50–61 Force 7, 62–74 Force 8, 75–88 Force 9, 89–102 Force 10, 103–117 Force 11 and 118 km/h or more Force 12. The converter on this page does this for km/h, mph, knots, m/s and ft/s.

At what Beaufort Force should small boats seek shelter?

There is no single legal limit; it depends on the boat, crew and sea room. Small open boats are commonly advised to stay in harbor from about Force 4–5, many keelboats reef from Force 5–6, and Force 7 and above is hazardous for most small craft. Always check the official marine forecast and local small-craft warnings before departing.

Beaufort Scale Reference

Look up all 13 Beaufort wind forces in knots, m/s, mph and km/h with wave height, sea state and land signs, and convert any wind speed to a force.

The Beaufort scale runs from Force 0 (calm, under 1 knot) to Force 12 (hurricane force, 64 knots or more).

Wind Speed to Beaufort Force

Band in knots / m/s
Band in mph / km/h
Probable (max) wave
Sea state

Beaufort Wind Force Scale (0–12)

10-minute mean wind at 10 m height. Wave heights are probable (maximum probable) values for well-developed wind waves in the open sea.

Force	Description	Knots	m/s	mph	km/h	Wave m (max)	Sea state	At sea	On land
0	Calm	< 1	0–0.2	< 1	< 1	–	0 · Calm (glassy)	Sea like a mirror.	Calm; smoke rises vertically.
1	Light air	1–3	0.3–1.5	1–3	1–5	0.1 (0.1)	1 · Calm (rippled)	Ripples with the appearance of scales; no foam crests.	Smoke drift shows wind direction; wind vanes do not move.
2	Light breeze	4–6	1.6–3.3	4–7	6–11	0.2 (0.3)	2 · Smooth (wavelets)	Small wavelets, still short but more pronounced; crests look glassy and do not break.	Wind felt on the face; leaves rustle; ordinary vanes move.
3	Gentle breeze	7–10	3.4–5.4	8–12	12–19	0.6 (1.0)	3 · Slight	Large wavelets; crests begin to break; glassy foam, perhaps scattered white horses.	Leaves and small twigs in constant motion; light flags extended.
4	Moderate breeze	11–16	5.5–7.9	13–18	20–28	1.0 (1.5)	3–4 · Slight to moderate	Small waves becoming longer; fairly frequent white horses.	Raises dust and loose paper; small branches move.
5	Fresh breeze	17–21	8.0–10.7	19–24	29–38	2.0 (2.5)	4 · Moderate	Moderate waves of a more pronounced long form; many white horses; chance of some spray.	Small trees in leaf begin to sway; crested wavelets on inland waters.
6	Strong breeze	22–27	10.8–13.8	25–31	39–49	3.0 (4.0)	5 · Rough	Large waves begin to form; white foam crests are more extensive; probably some spray.	Large branches in motion; whistling in telephone wires; umbrellas hard to use.
7	Near gale	28–33	13.9–17.1	32–38	50–61	4.0 (5.5)	5–6 · Rough to very rough	Sea heaps up; white foam from breaking waves begins to be blown in streaks along the wind.	Whole trees in motion; inconvenience felt when walking against the wind.
8	Gale	34–40	17.2–20.7	39–46	62–74	5.5 (7.5)	6–7 · Very rough to high	Moderately high waves of greater length; crest edges break into spindrift; well-marked foam streaks.	Twigs break off trees; progress on foot generally impeded.
9	Strong gale	41–47	20.8–24.4	47–54	75–88	7.0 (10.0)	7 · High	High waves; dense streaks of foam; crests begin to topple, tumble and roll over; spray may affect visibility.	Slight structural damage (chimney pots and slates removed).
10	Storm	48–55	24.5–28.4	55–63	89–102	9.0 (12.5)	8 · Very high	Very high waves with long overhanging crests; sea surface white with great patches of foam; heavy tumbling; visibility affected.	Seldom experienced inland; trees uprooted; considerable structural damage.
11	Violent storm	56–63	28.5–32.6	64–72	103–117	11.5 (16.0)	8 · Very high	Exceptionally high waves (small and medium ships may be lost to view behind them); sea covered with long white patches of foam.	Very rarely experienced; widespread damage.
12	Hurricane force	64+	32.7+	73+	118+	14.0+	9 · Phenomenal	Air filled with foam and spray; sea completely white with driving spray; visibility very seriously affected.	Devastation. 64 kn is also the Category 1 threshold on the Saffir–Simpson scale, which uses a 1-minute mean.

Sources: UK Met Office Beaufort wind force scale (knots, m/s, wave heights, sea-state code and terms) and WMO-No. 558 Manual on Marine Meteorological Services (specifications at sea). mph and km/h bands are the Met Office conversions. Sea state can lag the wind: waves take hours to build after the wind rises and persist after it drops.

Beaufort Number Formula and Worked Examples

The scale bands follow the empirical relation v = 0.836 × B^3/2 m/s (or 1.625 × B^3/2 knots), where v is the 10 m wind speed and B the Beaufort number. Solving for B gives a decimal Beaufort number: B = (v ÷ 0.836)^2/3.

25 knots = 25 × 0.5144 = 12.86 m/s → B = (12.86 ÷ 0.836)^2/3 = 6.2. The table band for 25 kn is Force 6, strong breeze.
40 mph = 17.88 m/s → B = 7.7. Rounded to whole mph, 40 mph falls in the 39–46 mph band: Force 8, gale.
Force 5 mean speed: 0.836 × 5^1.5 = 9.3 m/s = 18 kn, inside the 17–21 kn band.

The official force comes from the table bands, not the formula; near a band edge the two can differ by one. Conversions: 1 kn = 0.514444 m/s = 1.150779 mph = 1.852 km/h exactly.

Quick answer

The Beaufort scale runs from Force 0 (calm, under 1 knot) to Force 12 (hurricane force, 64 knots or more). Force 4 is 11–16 kn, Force 6 is 22–27 kn, Force 8 (gale) is 34–40 kn and Force 10 (storm) is 48–55 kn, with probable open-sea waves of 1, 3, 5.5 and 9 m.

Examples

Force	Knots	m/s	mph	Probable wave
4 Moderate breeze	11–16	5.5–7.9	13–18	1 m
6 Strong breeze	22–27	10.8–13.8	25–31	3 m
8 Gale	34–40	17.2–20.7	39–46	5.5 m
10 Storm	48–55	24.5–28.4	55–63	9 m
12 Hurricane force	64+	32.7+	73+	14+ m

Enter a wind speed in knots, mph, km/h or m/s to get its Beaufort force, sea state and wave height.

Summary

The Beaufort scale is an empirical 13-point wind force scale (0–12) devised by Francis Beaufort in 1805. Each force covers a band of 10-minute mean wind speed at 10 m height and the sea and land conditions that go with it. This reference lists every force in knots, m/s, mph and km/h with the WMO probable and maximum wave heights and the Met Office sea-state terms, and converts any measured speed to a force and a decimal Beaufort number (v = 0.836 B^1.5 m/s).

How it works

Enter a wind speed and pick its unit (knots, mph, km/h, m/s or ft/s).
The speed is rounded to the precision of that unit's official table (whole knots, mph or km/h; 0.1 m/s) and matched to its Beaufort band.
The result shows the force, name, the band in every unit, probable and maximum wave height, sea state and the decimal Beaufort number B = (v ÷ 0.836)^(2/3).
The matching row is highlighted in the full table, which you can filter by force, name or description.

Use cases

Interpret Beaufort wind forecasts when planning offshore or coastal passages.
Cross-reference anemometer readings with observable sea and land conditions.
Study for maritime officer or yacht master certification exams.
Brief crew on expected conditions using standardized Beaufort terminology.
Convert between knots, m/s, mph and km/h wind readings in the field.
Assess whether recorded wind observations match expected wave development.
Teach new sailors and weather enthusiasts wind force classifications.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu