Beaufort wind scale
Type a wind speed, or slide through the forces, to see the Beaufort number, its name and what that wind does to the sea and the land around you. The full table is below.
Beaufort force
5
Fresh breeze
- knots
- 17–21
- km/h
- 31–40
- mph
- 19–25
- m/s
- 9–11
At sea: Moderate, longer waves, plenty of whitecaps and a little spray. Waves about 2.0 m, up to 2.5 m on the open sea.
On land: Small trees in leaf begin to sway.
| Force | Name | Knots | km/h | mph | m/s | At sea | On land |
|---|---|---|---|---|---|---|---|
| 0 | Calm | under 1 | under 1 | under 1 | under 1 | The sea is flat and glassy, reflecting the sky.No waves to speak of | Smoke climbs straight up and nothing stirs. |
| 1 | Light air | 1–3 | 1–6 | 1–4 | 1–2 | Faint ripples like fish scales, with no foam.Waves about 0.1 m, up to 0.1 m | Drifting smoke shows the direction, but a weather vane stays put. |
| 2 | Light breeze | 4–6 | 6–12 | 4–7 | 2–3 | Short, small wavelets with glassy crests that do not break.Waves about 0.2 m, up to 0.3 m | You feel it on your face, leaves rustle and the vane starts to turn. |
| 3 | Gentle breeze | 7–10 | 12–19 | 7–12 | 4–5 | Bigger wavelets; crests start to break and the odd whitecap appears.Waves about 0.6 m, up to 1.0 m | Leaves and thin twigs keep moving and a light flag stands out. |
| 4 | Moderate breeze | 11–16 | 19–31 | 12–19 | 6–8 | Small waves growing longer, with whitecaps fairly often.Waves about 1.0 m, up to 1.5 m | Dust and loose paper lift off the ground; small branches sway. |
| 5 | Fresh breeze | 17–21 | 31–40 | 19–25 | 9–11 | Moderate, longer waves, plenty of whitecaps and a little spray.Waves about 2.0 m, up to 2.5 m | Small trees in leaf begin to sway. |
| 6 | Strong breeze | 22–27 | 40–51 | 25–32 | 11–14 | Larger waves form; whitecaps are common, with more spray.Waves about 3.0 m, up to 4.0 m | Big branches move and overhead wires whistle. |
| 7 | Near gale | 28–33 | 51–62 | 32–39 | 14–17 | The sea heaps up and foam from breaking waves blows in streaks.Waves about 4.0 m, up to 5.5 m | Whole trees sway and walking into the wind takes effort. |
| 8 | Gale | 34–40 | 62–75 | 39–47 | 17–21 | Longer, fairly high waves; crest edges break into spindrift.Waves about 5.5 m, up to 7.5 m | Twigs snap off trees and it is hard to make headway on foot. |
| 9 | Strong gale | 41–47 | 75–88 | 47–55 | 21–24 | High waves, dense streaks of foam, crests start to roll over; spray cuts visibility.Waves about 7.0 m, up to 10.0 m | Minor structural damage: roof tiles and slates may lift. |
| 10 | Storm | 48–55 | 88–103 | 55–64 | 25–28 | Very high waves with long overhanging crests; the sea looks white with foam.Waves about 9.0 m, up to 12.5 m | Rare inland: trees are uprooted and buildings are damaged. |
| 11 | Violent storm | 56–63 | 103–118 | 64–73 | 29–32 | Exceptionally high waves; patches of foam cover the sea and visibility drops further.Waves about 11.5 m, up to 16.0 m | Seldom felt inland; damage goes beyond force 10. |
| 12 | Hurricane force | 64+ | 118+ | 73+ | 33 and over | The air is full of foam and spray; the sea is completely white and visibility is very poor.Waves 14 m or more | Seldom felt inland; damage can be devastating. |
Weather safety: wind speeds in forecasts are averages, and gusts are stronger. For sailing, hill walking or anything exposed, check your weather service's warnings and the marine forecast.
Reading the scale
Francis Beaufort devised the scale in 1805 for sailors, who judged the wind by its effect on the sea. Each force covers a band of mean wind speeds, set in whole knots: force 4, a moderate breeze, runs from 11 to 16 knots. The knot limits, the speeds in metres per second, the names and the wave heights here follow the Met Office table. The Met Office calls force 9 a severe gale, though the official term is strong gale.
The calculator rounds your speed to the nearest whole knot before matching it, as the table is written. The km/h and mph columns are converted from the knot limits, counting each band from half a knot below its lower limit to half a knot above its upper one, then rounded.
Sea and land descriptions
The descriptions are written for this page and checked against the Met Office and NOAA tables. The wave heights are for well-developed waves on the open sea; the Met Office notes that the sea takes time to build after the wind gets up, so early in a blow the waves are smaller.
Sources
- Met Office. Beaufort wind force scale.
- NOAA Storm Prediction Center. Beaufort Wind Scale (effects on water and land).
- NIST Special Publication 811, Appendix B.9: conversion factors.
Questions
Is a gale the same as force 8?
Yes. Force 8, a gale, is 34 to 40 knots, about 39 to 47 mph or 62 to 75 km/h in the table above. Force 9 is a strong (or severe) gale.
What is hurricane force?
Force 12, the top of the scale: a mean wind of 64 knots or more, which is about 74 mph or 119 km/h.
Do forecasts use the Beaufort scale?
The Met Office uses it in its marine forecasts. Most land forecasts give speeds in mph, km/h or m/s, which you can turn into a force here or on the wind speed converter.