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Beaufort scale

The Beaufort scale describes the force of the wind in thirteen steps, from 0 (calm) to 12 (hurricane force). Each force is a range of wind speeds; the thresholds follow the World Meteorological Organization (WMO). Type a speed in the converter or read the ranges from the table, then check yourself in the quiz.

Force 6 Strong breeze Large waves, white foam crests, some spray.

25 kn = 12.9 m/s = 46.3 km/h

The force is read from the thresholds of the unit you typed in (the WMO table has separate columns for knots, m/s and km/h). A link with ?v= and ?unit= opens a ready value.

The Beaufort scale table

Force Name and look of the sea Knots m/s km/h
0 Calm Sea like a mirror; sails hang limp. <1 0.0-0.2 <1
1 Light air Ripples like scales, without foam crests. 1-3 0.3-1.5 1-5
2 Light breeze Small, short wavelets with glassy crests. 4-6 1.6-3.3 6-11
3 Gentle breeze Crests begin to break; scattered white horses. 7-10 3.4-5.4 12-19
4 Moderate breeze Waves getting longer; numerous white horses. 11-16 5.5-7.9 20-28
5 Fresh breeze Moderate waves, white horses everywhere, occasional spray. 17-21 8.0-10.7 29-38
6 Strong breeze Large waves, white foam crests, some spray. 22-27 10.8-13.8 39-49
7 Near gale Sea heaps up; foam blows in streaks with the wind. 28-33 13.9-17.1 50-61
8 Gale Moderately high, long waves; crests break into spindrift. 34-40 17.2-20.7 62-74
9 Strong gale High waves, dense streaks of foam, spray starts to reduce visibility. 41-47 20.8-24.4 75-88
10 Storm Very high waves with overhanging crests; the sea turns white. 48-55 24.5-28.4 89-102
11 Violent storm Exceptionally high waves; sea covered with foam, visibility reduced. 56-63 28.5-32.6 103-117
12 Hurricane force Air filled with foam and spray; sea completely white. 64+ 32.7+ 118+

Speed ranges per WMO (knots, m/s) and the canonical km/h table; force names are the official WMO terms. The sea descriptions are our own editorial based on the Beaufort table: they help estimate the wind by eye and are not the definition of a force.

Sea state: the Douglas scale

Beaufort describes the wind; sea state (wave height) is described by the Douglas scale from 0 to 9. It is the Douglas figure you will find in the logbook next to the wind force.

Degree Sea state Wave height [m]
0 Calm (glassy) 0
1 Calm (rippled) 0-0.1
2 Smooth (wavelets) 0.1-0.5
3 Slight 0.5-1.25
4 Moderate 1.25-2.5
5 Rough 2.5-4
6 Very rough 4-6
7 High 6-9
8 Very high 9-14
9 Phenomenal 14 and above

Wave-height ranges and terms per the Douglas scale (WMO code 3700). The height is the significant wave height.

Frequent questions about the Beaufort scale

From which Beaufort force is it a gale?

From force 8, which starts at 34 knots (about 62 km/h). Above it come strong gale (force 9), storm (force 10), violent storm (force 11) and hurricane force (force 12, from 64 knots).

How do I convert Beaufort forces to km/h or m/s?

The scale is ordinal: each force is a range of speeds, not a single value (force 6, for example, is 22-27 knots, i.e. 39-49 km/h). For rough estimates there is the formula v = 0.836·B^1.5 m/s, but in practice you read the thresholds from the table.

Does the Beaufort scale describe wave height?

No. Beaufort describes the force of the wind (the look of the sea is only a guide for estimating it). Sea state, i.e. wave height, is described by the separate Douglas scale from 0 to 9; its table is further down this page.

Where does the Beaufort scale come from?

It was devised in 1805 by Francis Beaufort, a Royal Navy officer, initially as a description of how a sailing ship behaved in different winds. The version in force today has wind-speed thresholds set by the World Meteorological Organization (WMO).

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