About the Beaufort Scale Converter
This Beaufort scale converter translates a wind speed into a Beaufort force number from 0 (calm) to 12 (hurricane force), with the official description, the typical open-sea wave height, and what you would see at sea and on land. It also works the other way: pick a force from a shipping or marine forecast and see the matching speed range in knots, mph, km/h and m/s.
Sailors, kayakers, kitesurfers, anglers and coastal walkers use the Beaufort scale because marine forecasts (such as the UK Shipping Forecast) give wind in forces, while apps and anemometers give speeds. It is also handy for drone pilots, outdoor event planners and anyone curious what "gale force 8" actually means. The full table below the result lists every force.
Speeds are classified using the World Meteorological Organization knot bands, after rounding to the nearest knot as marine reports do, so a speed right on a boundary in another unit can fall either side. Wave heights are probable values in the open sea well away from land; near coasts, in shallow water or with strong tides, real waves can be very different.
How to use the beaufort scale converter
- 1Choose whether you are converting a speed or a Beaufort force.
- 2Enter the wind speed and its unit, or pick a force from 0 to 12.
- 3Read the force name, speed ranges in every unit and the wave height.
- 4Check the sea and land descriptions to match what you see outside.
Formula and method
The Beaufort scale was devised in 1805 by Francis Beaufort, then a Royal Navy commander and later an admiral, from the sails a warship could carry, and later tied to measured wind speeds at 10 m above the surface. The World Meteorological Organization defines each force as a band of mean wind speed in knots (force 4 is 11–16 knots, force 8 is 34–40 knots and so on), and the bands follow the empirical relation v = 0.836·B^1.5 m/s.
In speed mode the calculator converts your speed to knots, rounds to the nearest knot and looks up the band; it also shows the continuous Beaufort number from the empirical formula for interest. Wave heights are the WMO probable heights for the open sea, with the probable maximum in the table. Gusts can be 30–50% stronger than the mean wind used for the force.
- B
- Beaufort number (force)
- v
- Mean wind speed at 10 m height (m/s)
Worked examples
15 knots on the boat's anemometer
15 knots falls in the 11–16 knot band, so it is force 4, a moderate breeze with about 1 m waves. 15 knots = 7.72 m/s, and (7.72 ÷ 0.836)^⅔ = 4.40 on the continuous scale.
Weather app says 25 mph
25 mph = 21.7 knots, which rounds to 22 knots – the bottom of the 22–27 knot band – so it is force 6, a strong breeze with large branches moving.
70 km/h wind
70 km/h = 37.8 knots, rounding to 38, inside the 34–40 knot gale band. Expect around 5.5 m waves in the open sea.
35 m/s sustained wind in a tropical cyclone
35 m/s = 68 knots of mean (not gust) wind, above the 64-knot threshold, so it is force 12 – hurricane force – the top of the scale.
Shipping forecast says force 7
Force 7, near gale, means mean winds of 28–33 knots (50–61 km/h, 32–38 mph) with the sea heaping up and probable waves of about 4 m.
Frequently asked questions
What is the Beaufort scale?+
It is a 0–12 scale of wind force based on observed effects at sea and on land. Each force corresponds to a band of mean wind speed measured 10 m above the surface, from calm (under 1 knot) to hurricane force (64 knots and above).
What wind speed is gale force?+
Gale force 8 is 34–40 knots (62–74 km/h or 39–46 mph). Force 9, a strong or severe gale, is 41–47 knots, and force 10, a storm, is 48–55 knots.
Is a force 5 wind safe for sailing?+
Force 5 (17–21 knots) is a fresh breeze with many white horses and about 2 m waves offshore. Experienced crews in seaworthy boats handle it routinely, but it is demanding for beginners, dinghies and small open boats.
How do I convert knots to Beaufort?+
Compare the speed with the knot bands: under 1 = 0, 1–3 = 1, 4–6 = 2, 7–10 = 3, 11–16 = 4, 17–21 = 5, 22–27 = 6, 28–33 = 7, 34–40 = 8, 41–47 = 9, 48–55 = 10, 56–63 = 11 and 64 or more = 12.
Are there Beaufort forces above 12?+
The WMO scale stops at 12. Some countries, such as China and Taiwan, use extended forces 13–17 for typhoons, but elsewhere winds above force 12 are graded with tropical cyclone scales such as the Saffir–Simpson hurricane scale used in the Americas.