The Beaufort scale turns a wind speed into a number that describes what the sea is doing, which is why it survived two centuries of better instruments. Here it is in knots, force by force, with the part the standard tables leave out: what each force does to a boat you are racing rather than to a merchant ship.
The scale in knots
| Force | Term | Knots | Sea |
|---|---|---|---|
| 0 | Calm | 0 | Glassy |
| 1 | Light air | 1 to 3 | Ripples |
| 2 | Light breeze | 4 to 6 | Small wavelets |
| 3 | Gentle breeze | 7 to 10 | Large wavelets, scattered white horses |
| 4 | Moderate breeze | 11 to 16 | Small waves, frequent white horses |
| 5 | Fresh breeze | 17 to 21 | Moderate waves, many white horses |
| 6 | Strong breeze | 22 to 27 | Large waves, extensive foam crests |
| 7 | Near gale | 28 to 33 | Sea heaps up, foam blows in streaks |
| 8 | Gale | 34 to 40 | Moderately high waves, crests break into spindrift |
| 9 | Strong gale | 41 to 47 | High waves, dense foam, spray affects visibility |
| 10 | Storm | 48 to 55 | Very high waves, sea appears white |
| 11 | Violent storm | 56 to 63 | Exceptionally high waves, visibility reduced |
| 12 | Hurricane | Over 63 | Air filled with foam and spray, sea completely white |
Note that the Met Office publishes a single mean speed for each force while the standard scale publishes a range. Both are correct. The range is the one to use when you are reading an instrument, because your instrument reports an instantaneous speed, not an average.
Force 0 to 3: the forces that decide whether there is a race at all
Below force 2 most classes cannot hold a course, and the question stops being tactical and becomes whether the race committee can set a fair line at all. Force 3 is where a dinghy fleet starts sailing properly: full sails, crew weight inboard, and a line bias that finally matters.
The trap in this range is that the wind at deck level and the wind at the top of the rig are different, and the gradient is steepest in light air. A reading taken sitting down in the boat can be two knots under what the sails are seeing.
Force 4 to 5: the best racing and the first hard decisions
Force 4, 11 to 16 knots, is where most dinghy and keelboat racing is at its best. Force 5, 17 to 21 knots, is where fleets start to separate by skill rather than by boat speed, and where the first capsizes and the first gear failures appear.
This is also the band where a measured number stops being a nice-to-have. Between 17 and 21 knots the difference between running another race and sending the fleet in is a judgement call, and a judgement call made on a number is far easier to defend afterwards than one made on how it felt on the committee boat. Our guide on how to choose an anemometer for measuring wind speed covers what actually matters in an instrument you will be reading in spray.
Force 6 to 7: where racing usually stops
Force 6, 22 to 27 knots, is the upper limit for most junior and club fleets and for a good many senior ones. Force 7, 28 to 33 knots, is where a race committee is no longer running a race, it is running a recovery operation.
There is no rule that names a wind speed. The decision to abandon sits with the race committee, and it rests on the fleet, the water and the safety cover rather than on the number alone. We have written at length about what wind speed is too high for a sailing race and how to build that call on evidence.
Force 8 and above
Force 8 upwards is not a racing conversation. It is a mooring, shelter and safety-boat conversation. The scale keeps going to force 12 because it was built for ships at sea, not for a dinghy fleet on a lake.
Reading the scale on the water
Two practical points. First, the sea descriptions in the table lag the wind by a long way in enclosed water. In a small bay you can have force 6 wind over a force 3 sea, and if you estimate force from the sea alone you will underread badly.
Second, gusts are not the average. A handheld instrument that shows an instantaneous reading will show the gust, and a fleet is capsized by the gust rather than by the mean. When you record a number for the race record, record both the average and the highest gust you saw.
A cup anemometer suits this better than a vane instrument for exactly this reason: it responds to wind from any direction without being pointed, so you can take a reading with one hand while the boat is moving. Our handheld anemometers are built around that use, with a three-cup rotor and a measuring head that can be replaced if it takes a knock.
Frequently asked questions
What is force 5 on the Beaufort scale in knots?
Force 5 is a fresh breeze, 17 to 21 knots, with moderate waves and many white horses.
At what Beaufort force do sailing races usually get abandoned?
There is no fixed number in the rules. In practice most club and junior fleets stop somewhere in force 6, 22 to 27 knots, and the decision belongs to the race committee based on the fleet and the safety cover available.
Is the Beaufort scale measured in knots or mph?
The official scale is defined in knots. Conversions to mph and km/h exist but the knot values are the primary ones, which is why marine forecasts and race documents use them.
Why does the Beaufort scale go to 12 if boats never sail in force 12?
It was written in 1805 for naval ships describing the state of the sea, not for small boat racing. The upper forces describe survival conditions at sea rather than anything raceable.
In short
Force 4 and 5 are the racing band, force 6 is the edge, and everything above it is a safety decision. The scale tells you what the number means. Getting the number itself right, gust as well as average, is the part that needs an instrument rather than an estimate.