Beaufort Wind Force Scale
The Beaufort scale rates wind from force 0 (calm, under 1 mph) to force 12 (hurricane force, 73 mph / 64 knots and above). Gale force is force 8: 39–46 mph or 34–40 knots.
Wind speed to Beaufort force
Source: NOAA National Weather Service: Beaufort wind force scale · All sources
Enter a wind speed to find its Beaufort number, or use the chart to estimate wind from what you can see: smoke, leaves, trees and the state of the sea.
Beaufort scale chart
Speeds are the mean wind measured 10 meters above open ground. Gusts can be 30–50% stronger. Wave heights are typical values in the open sea, far from land.
| Force | Description | mph | knots | km/h | m/s | Wave height (m) | On land | At sea |
|---|---|---|---|---|---|---|---|---|
| 0 | Calm | < 1 | < 1 | < 1 | 0–0.2 | 0 | Smoke rises vertically. | Sea like a mirror. |
| 1 | Light air | 1–3 | 1–3 | 1–5 | 0.3–1.5 | 0.1 | Smoke drifts; wind vanes don't move. | Ripples, no foam crests. |
| 2 | Light breeze | 4–7 | 4–6 | 6–11 | 1.6–3.3 | 0.2 | Wind felt on the face; leaves rustle; vanes move. | Small wavelets; crests glassy, not breaking. |
| 3 | Gentle breeze | 8–12 | 7–10 | 12–19 | 3.4–5.4 | 0.6 | Leaves and twigs in constant motion; light flags extend. | Large wavelets; crests begin to break; scattered whitecaps. |
| 4 | Moderate breeze | 13–18 | 11–16 | 20–28 | 5.5–7.9 | 1 | Dust and loose paper lifted; small branches move. | Small waves becoming longer; fairly frequent whitecaps. |
| 5 | Fresh breeze | 19–24 | 17–21 | 29–38 | 8.0–10.7 | 2 | Small trees in leaf begin to sway. | Moderate, longer waves; many whitecaps; some spray. |
| 6 | Strong breeze | 25–31 | 22–27 | 39–49 | 10.8–13.8 | 3 | Large branches move; wires whistle; umbrellas hard to use. | Large waves form; white foam crests everywhere; some spray. |
| 7 | Near gale | 32–38 | 28–33 | 50–61 | 13.9–17.1 | 4 | Whole trees in motion; walking against the wind is hard. | Sea heaps up; foam blown in streaks. |
| 8 | Gale | 39–46 | 34–40 | 62–74 | 17.2–20.7 | 5.5 | Twigs break off trees; progress on foot is impeded. | Moderately high waves; crests break into spindrift. |
| 9 | Strong gale | 47–54 | 41–47 | 75–88 | 20.8–24.4 | 7 | Slight structural damage; chimney pots and slates removed. | High waves; dense foam streaks; spray may reduce visibility. |
| 10 | Storm | 55–63 | 48–55 | 89–102 | 24.5–28.4 | 9 | Trees uprooted; considerable structural damage. Rare inland. | Very high waves with overhanging crests; sea looks white. |
| 11 | Violent storm | 64–72 | 56–63 | 103–117 | 28.5–32.6 | 11.5 | Widespread damage. Very rare. | Exceptionally high waves; sea covered in foam. |
| 12 | Hurricane force | 73+ | 64+ | 118+ | 32.7+ | 14+ | Devastation. | Air filled with foam and spray; sea completely white. |
How to estimate wind without an instrument
The scale was built for people without anemometers, and it still works:
- Smoke and flags. Smoke rising straight up is force 0. Smoke drifting while a flag hangs limp is force 1. A light flag standing out is force 3.
- Trees. Leaves rustling is force 2; twigs constantly moving is 3; small branches swaying is 4; small trees swaying is 5; large branches moving is 6; whole trees moving is 7.
- Walking. When walking against the wind becomes hard, you're at force 7 or more.
- Water. Scattered whitecaps on open water mean about force 3; many whitecaps mean force 5.
Converting between the units
1 knot = 1.15078 mph = 1.852 km/h = 0.51444 m/sEach unit has its own published band, rounded to whole numbers, so the band edges don't line up perfectly across units. The converter above uses the band for the unit you enter. The Beaufort number is related to speed by an approximate formula, v = 0.836 × B3/2 m/s, which is how the bands were standardized in the 20th century.
History
Francis Beaufort wrote his first version in 1805, describing each force by how much sail a Royal Navy frigate could carry. It became the Navy's standard in 1838. When steam replaced sail, the descriptions were rewritten around sea state, and in the 1900s meteorologists matched each force to measured wind speeds. The World Meteorological Organization keeps the international version used by weather services today.
Frequently asked questions
What does force 6 mean on the Beaufort scale?
Force 6 is a strong breeze: 25–31 mph, 22–27 knots or 39–49 km/h. Large branches move, wires whistle and umbrellas are hard to use. At sea, large waves form with white foam crests.
What wind speed is gale force?
A gale is force 8: 39–46 mph, 34–40 knots or 62–74 km/h. A strong gale (force 9) is 47–54 mph.
What is the highest number on the Beaufort scale?
12, hurricane force, for winds of 64 knots (73 mph, 118 km/h) and above. Some countries, including China, use an extended scale to force 17 for typhoons.
Who invented the Beaufort scale?
Francis Beaufort, an officer in Britain's Royal Navy, devised it in 1805 to describe wind by its effect on a sailing ship's sails. The Royal Navy made it standard in 1838, and descriptions for land and sea were added later.
Is the Beaufort scale the same as the hurricane categories?
No. The Beaufort scale stops at force 12, which is where hurricanes begin. The Saffir-Simpson scale then rates hurricanes from Category 1 (74–95 mph) to Category 5 (157 mph and above).
Sources
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- Last updated
- November 5, 2026
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