Beaufort scale calculator
Enter a wind speed, get the Beaufort force.
What is the Beaufort scale?
A scale of wind force from 0 (calm) to 12 (hurricane force), defined by wind speed. The calculator uses the knot ranges published by NOAA’s Storm Prediction Center and the UK Met Office.
What wind speed is a gale?
Force 8, a gale, is 34 to 40 knots — about 39 to 46 mph. Force 7 (near gale) is 28 to 33 knots and force 9 (strong gale) 41 to 47 knots.
When is wind hurricane force?
Force 12 is 64 knots or more — about 74 mph and up.
How do knots convert to mph?
One knot is one nautical mile an hour: exactly 1.852 km/h, or about 1.15 mph. The Met Office rounds this to “multiply by 1.15” for mph and 0.514 for meters per second.
How does the calculator pick the force?
It converts your speed to knots and rounds to the nearest whole knot, then finds the force whose range contains it.
What do the forces look like on land?
The scale was developed in 1805 by Sir Francis Beaufort of the Royal Navy, who tied each force to what the wind does. NOAA’s Storm Prediction Center lists the land effects: at force 4 (11–16 knots) dust, leaves and loose paper are lifted and small branches move; at force 6 (22–27 knots) larger branches move and wires whistle; at force 7 (28–33 knots) whole trees move and walking against the wind takes effort; at force 8 (34–40 knots) twigs break off trees and progress is generally impeded; at force 9 (41–47 knots) slight structural damage occurs; at force 10 (48–55 knots) trees are broken or uprooted and considerable structural damage occurs, although this is seldom experienced on land. Force 12 begins at 64 knots.
Worked example: converting mph to a force number
Wind forecast at 25 mph. Divide by 1.1508 (miles per hour in one knot) to get 21.7 knots, which rounds to 22. That falls in the 22–27 knot range, so it is force 6, a strong breeze. At 20 mph the speed is 17.4 knots, rounded to 17, which is force 5, a fresh breeze (17–21 knots), and 45 mph is 39.1 knots, rounded to 39: force 8, a gale. The calculator rounds to the nearest whole knot first, so 16.4 knots counts as 16 (force 4) while 16.5 knots counts as 17 (force 5).
Common mistakes with the Beaufort scale
Reading knots as miles per hour: a wind of 30 knots is force 7 (near gale), but 30 mph is only about 26 knots, force 6 (strong breeze). Treating the descriptions as exact: the land and sea effects are rough guides, and the speed ranges are what define each force. Comparing a gust with a sustained wind without saying which one a speed refers to. And expecting an upper limit: force 12 only says 64 knots or more, so the scale cannot separate a strong hurricane from a weaker one.
Sources and related calculators
Sources: NOAA National Weather Service Storm Prediction Center, “The Beaufort Wind Scale” (force numbers, knot ranges, land and sea descriptions, origin of the scale in 1805); UK Met Office, Beaufort scale (the knot-to-mph rule quoted above). Related: wind chill calculator, heat index calculator, speed converter and dew point calculator.