The Beaufort Scale for Sailors — Force 0 to 12 Explained

The Beaufort scale describes wind strength from force 0 (calm, under 1 knot) to force 12 (hurricane force, 64 knots or more) by its effect on the sea. Force 4 (11 to 16 knots) is a good sailing breeze, force 6 (22 to 27 knots) is often called a 'yachtsman's gale', and force 8 (34 to 40 knots) is a gale. Forecasts give mean wind; gusts can be much stronger.

A rough sea of breaking waves and spray under a dark sky — Breaking seas in strong winds. The Beaufort scale is defined by what the wind does to the sea. Photo: JOHN TOWNER heytowner, CC0.

Beaufort scale with mean wind speed and probable wave height in the open sea (Met Office).
Force	Name	Knots	Waves	On a 10 m yacht
0	Calm	Under 1	Flat	Motor
1	Light air	1–3	0.1 m	Drifting
2	Light breeze	4–6	0.2 m	Gentle sailing
3	Gentle breeze	7–10	0.6 m	Ideal for learning
4	Moderate breeze	11–16	1 m	Great sailing; small boats reef
5	Fresh breeze	17–21	2 m	Reef; wet upwind
6	Strong breeze	22–27	3 m	Two reefs; experienced crews
7	Near gale	28–33	4 m	Deep reefs; stay in harbour unless prepared
8	Gale	34–40	5.5 m	Do not set out
9	Severe gale	41–47	7 m	Storm tactics
10	Storm	48–55	9 m	Dangerous
11	Violent storm	56–63	11.5 m	Dangerous
12	Hurricane force	64+	14 m+	Extreme

Bar chart of the Beaufort scale from force 0 to 12 in knots, coloured from learning conditions through great sailing to do not set out — The mean wind for each force, coloured by what it means for a 10 m cruising yacht.

Mean wind vs gusts

Forecast forces are average speeds. Gusts can be 30 to 50 per cent stronger, more in showers and off high land. Reef for the gusts you expect, not the average.

Why the sea matters as much as the wind

Beaufort's wave heights are for the open sea after the wind has blown for a long time over a long fetch. Close to a weather shore the sea will be flatter. Against a strong tidal stream, over shallows or off headlands, the same wind can make short, steep, breaking seas.

See each force visualised with the interactive Beaufort scale and keep the Beaufort cheat sheet by the chart table.

Related guides

Frequently asked questions

What force is a gale?

Force 8: a mean wind of 34 to 40 knots. A gale warning is issued when force 8 or more, or gusts of 43 knots or more, are expected.

Is force 5 too windy for beginners?

Often, yes. Force 5 (17 to 21 knots) is manageable for a reefed cruising yacht with a competent crew but can be uncomfortable and tiring for new crew.

Start revising

← All guides Start the free theory course →