What is the Beaufort scale?

The Beaufort scale is a 0 to 12 measure of wind force based on observed effects on the sea and on land. It was devised by Francis Beaufort in 1805 and is now defined by the World Meteorological Organization with a specific wind-speed range for each force.

How is force calculated from wind speed?

The empirical relation is v equals 0.836 times B to the power 1.5, where v is wind speed in metres per second and B is the Beaufort number. Inverting it gives B equals the speed over 0.836 raised to the two-thirds power, which this tool rounds to the nearest force.

What wave heights does the table show?

The probable wave heights are the WMO open-sea figures for a fully developed sea well away from land. In sheltered water or where the fetch is short, the waves for a given force will be noticeably lower.

At what height is wind speed measured?

Beaufort wind speeds are referenced to the standard meteorological height of 10 metres above the surface. Wind measured lower down, such as at deck level, will read lighter than the equivalent Beaufort speed.

When does a gale warning apply?

A gale begins at Beaufort force 8, which is 34 knots and above. Force 9 is a strong gale, force 10 a storm, force 11 a violent storm, and force 12 a hurricane at 64 knots and above. Forecasters issue gale warnings from force 8 upward.

Beaufort Scale Wind & Sea State Reference

The Beaufort scale turns a number from 0 to 12 into a precise picture of the wind and the sea it raises. This reference lets you go either way: enter a measured wind speed to get the force, or scan the full table to read across force number, speed in three units, and probable wave height.

How it works

Each Beaufort force is defined by a wind-speed range in knots, which this tool converts to metres per second and kilometres per hour using 1 knot = 0.514444 m/s = 1.852 km/h. To turn a measured speed back into a force it uses the standard empirical relation:

v = 0.836 × B^1.5      (v in m/s, B = Beaufort number)
B = (v / 0.836)^(2/3)  (rounded to the nearest whole force)

The probable wave heights are the WMO open-sea values, describing a fully developed sea far from land.

Example and notes

A measured 25-knot wind is about 12.9 m/s, which the formula places at Beaufort force 6, a strong breeze with large waves and extensive foam crests. Remember that the wave-height column assumes deep, open water with unlimited fetch: in a harbour, a river mouth, or with an offshore wind, the sea will be far smoother than the table suggests for the same force. Always treat the sea-state description as the most reliable field cue when no anemometer is available.