Beaufort Scale Calculator
Type a wind speed in any unit and see its Beaufort force, how the sea looks, how high the waves get and what it means for boats and ships.
Wind speed to Beaufort force
Force 5Fresh breeze
17 to 21 knots, 19 to 24 mph, 31 to 39 km/h, 8.5 to 11 m/s
- Sea
- Moderate, longer waves. Many whitecaps and some spray.
- Wave height
- 2 m (7 ft), up to 2.5 m (8 ft)
- On board
- Small boats reef and take spray on deck. A cruise ship moves only slightly.
| Force | Name | Knots | mph | km/h | Wave height |
|---|---|---|---|---|---|
| 0 | Calm | 0 | 0 | 0 | Flat |
| 1 | Light air | 1 to 3 | 1 to 4 | 1 to 6 | 0.1 m |
| 2 | Light breeze | 4 to 6 | 5 to 7 | 7 to 12 | 0.2 m |
| 3 | Gentle breeze | 7 to 10 | 8 to 12 | 13 to 19 | 0.6 m |
| 4 | Moderate breeze | 11 to 16 | 13 to 18 | 20 to 30 | 1 m |
| 5 | Fresh breeze | 17 to 21 | 19 to 24 | 31 to 39 | 2 m |
| 6 | Strong breeze | 22 to 27 | 25 to 31 | 40 to 50 | 3 m |
| 7 | Near gale | 28 to 33 | 32 to 38 | 51 to 62 | 4 m |
| 8 | Gale | 34 to 40 | 39 to 46 | 63 to 75 | 5.5 m |
| 9 | Strong gale | 41 to 47 | 47 to 54 | 76 to 87 | 7 m |
| 10 | Storm | 48 to 55 | 55 to 63 | 88 to 102 | 9 m |
| 11 | Violent storm | 56 to 63 | 64 to 73 | 103 to 117 | 11.5 m |
| 12 | Hurricane | 64+ | 74+ | 118+ | 14 m |
Each force is the WMO band for the speed rounded to the nearest knot. Wave heights are the probable height in the open sea, lower near the coast. More history and examples in our Beaufort scale guide.
How to Find the Beaufort Force
From a forecast or a wind meter
Enter the wind speed
Type the sustained wind speed from a marine forecast, a weather app or an anemometer.
Pick the unit
Knots, mph, km/h or m/s. The speed is converted to knots and rounded to the nearest knot.
Read the sea state
See the force, its name, the probable wave height and what boats and ships do at that strength.
The Beaufort Scale in One Place
Force 0 to 12, as the WMO defines it
WMO force bands
Each force uses the World Meteorological Organization band in knots, the unit marine forecasts use.
Every speed unit
Ranges are shown in knots, mph, km/h and m/s, so a forecast in any unit reads the same.
Waves in metres and feet
The probable wave height in the open sea, and the probable maximum, for every force.
Free
No account and no limit. It runs in your browser, so it keeps working offline once the page has loaded.
Who Uses the Beaufort Scale
Sailors and boaters
Turn a forecast wind into a sea state before leaving harbour, and decide when to reef or stay in.
Cruise passengers
See what force 7 or force 8 means for the ship, and why a tender port might be skipped.
Students and weather watchers
Learn the scale for a navigation course, a geography class or a forecast in Beaufort.
Frequently Asked Questions
Everything you need to know about the Beaufort Scale Calculator.
What is the Beaufort scale?
The Beaufort scale describes wind strength from force 0, calm, to force 12, hurricane, by its effect on the sea. Admiral Francis Beaufort drew it up in 1805, and marine forecasts still use it.
How many knots is force 6?
Force 6, a strong breeze, is 22 to 27 knots, about 25 to 31 mph. The probable wave height in the open sea is 3 m (10 ft).
What wind speed is a gale?
A gale is force 8, 34 to 40 knots (39 to 46 mph). Force 9 is a strong gale at 41 to 47 knots.
How does the calculator pick the force?
It converts your speed to knots, rounds it to the nearest whole knot and finds the WMO band it falls in. The ranges in mph, km/h and m/s are the values that land in each band, so they can differ by one from tables that convert in another way.
Are the wave heights exact?
No. They are the probable heights in the open sea once the wind has blown for a while. Near the coast, in sheltered water or right after the wind rises, waves are lower.
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