Beaufort Wind Scale Converter
Convert any wind speed (m/s, km/h, mph or knots) to its Beaufort force number and description, or enter a force from 0 to 12 to see the matching speed range.
Convert Wind Speed to a Beaufort Force
Beaufort Wind Scale Formula
- \(v_{\text{m/s}}\) — mean wind speed converted to metres per second, measured 10 m above the ground
- \(t_n\) — the lower speed threshold for force \(n\): 0.3, 1.6, 3.4, 5.5, 8.0, 10.8, 13.9, 17.2, 20.8, 24.5, 28.5, 32.7 m/s for forces 1 to 12
- \(B\) — the Beaufort force number, from 0 (calm) to 12 (hurricane force)
Speeds in km/h, mph or knots are first converted to m/s (divide km/h by 3.6, mph by 2.237, knots by 1.944) before the thresholds are applied.
Beaufort Wind Scale – Calculation Example
Example: the mean wind is 12 m/s, measured 10 m above the ground.
\[ v = 12\ \text{m/s} \ge t_6 = 10.8\ \text{m/s}, \quad 12 < t_7 = 13.9\ \text{m/s} \Rightarrow B = 6 \]
Result: Beaufort force 6 — Strong breeze. Corresponding range 10.8–13.8 m/s (21–27 kn). On land: large branches in motion; umbrellas used with difficulty.
How to use the calculator
- Choose the Input type: “Wind speed” to convert a measured speed into a Beaufort force, or “Beaufort force number” to go the other way.
- If you chose wind speed, enter the value in Wind speed (v) and pick the unit — m/s, km/h, mph or knots.
- If you chose force number, enter a whole number from 0 to 12 in Beaufort force.
- Read the result: the Beaufort force with its name, the corresponding speed range, and what you would observe on land and at sea.
What is the Beaufort wind scale?
The Beaufort scale was introduced in 1805 by Royal Navy officer Francis Beaufort as a way to estimate wind strength from visual observations — originally the effect of the wind on a frigate’s sails. Long before anemometers became common, sailors could record wind conditions in a consistent, comparable way simply by describing what they saw. The scale runs from force 0 (calm, smoke rises vertically) to force 12 (hurricane force, widespread devastation).
Today the World Meteorological Organization (WMO) anchors each force number to a defined band of wind speed, measured as a mean value 10 metres above the ground. That makes the scale a bridge between what you observe and what instruments measure: a weather report saying “force 7, near gale” tells you the mean wind lies between 13.9 and 17.1 m/s (28–33 kn) without quoting a single number.
Because each force also carries standard descriptions of conditions on land and at sea, the scale remains useful for quick risk assessment. Force 6 is when umbrellas become difficult to use; force 8 (gale) breaks twigs off trees and impedes walking; force 10 (storm) uproots trees. Marine and aviation forecasts, especially in Europe, still quote winds in Beaufort forces or in the closely related knots, so knowing how to move between the two is a practical skill.
Note that the scale describes mean wind, not gusts. A gust of 20 m/s during a storm can occur while the mean wind sits comfortably in force 8. For safety decisions, always check both the mean wind and the gust forecast.
The full Beaufort scale
| Force | Name | m/s | km/h | mph | knots |
|---|---|---|---|---|---|
| 0 | Calm | 0–0.2 | <1 | <1 | <1 |
| 1 | Light air | 0.3–1.5 | 1–5 | 1–3 | 1–3 |
| 2 | Light breeze | 1.6–3.3 | 6–11 | 4–7 | 4–6 |
| 3 | Gentle breeze | 3.4–5.4 | 12–19 | 8–12 | 7–10 |
| 4 | Moderate breeze | 5.5–7.9 | 20–28 | 13–18 | 11–16 |
| 5 | Fresh breeze | 8.0–10.7 | 29–38 | 19–24 | 17–21 |
| 6 | Strong breeze | 10.8–13.8 | 39–49 | 25–31 | 22–27 |
| 7 | Near gale | 13.9–17.1 | 50–61 | 32–38 | 28–33 |
| 8 | Gale | 17.2–20.7 | 62–74 | 39–46 | 34–40 |
| 9 | Strong gale | 20.8–24.4 | 75–88 | 47–54 | 41–47 |
| 10 | Storm | 24.5–28.4 | 89–102 | 55–63 | 48–55 |
| 11 | Violent storm | 28.5–32.6 | 103–117 | 64–72 | 56–63 |
| 12 | Hurricane force | ≥ 32.7 | ≥ 118 | ≥ 73 | ≥ 64 |
Assumptions and limitations
- The calculator uses the official WMO speed limits, which are defined in m/s; the km/h, mph and knot bands are rounded equivalents, so a speed near a boundary may land on a different force depending on the unit you enter.
- The scale refers to the mean wind averaged over 10 minutes at 10 m height. It says nothing about gusts, which can be 30–50% higher, or about winds higher above the ground.
- Forces above 12 are not defined in the WMO scale. Hurricane categories (Saffir–Simpson) and tornado ratings (EF-scale) measure something different — storm intensity or damage — not mean surface wind.
- The land and sea descriptions are qualitative guides. Actual effects depend on terrain, buildings, exposure and sea state.
Related calculators
Wind speed also drives how cold and how hot it feels. To account for that, try the Wind Chill Calculator for winter conditions, the Heat Index Calculator for hot, humid weather, and the Dew Point Calculator for humidity context. Since wind speed depends on air properties, the Air Density Calculator is a useful companion for meteorology work.
Frequently Asked Questions
How do I find the Beaufort force by hand without a calculator?
Convert your speed to m/s (divide km/h by 3.6, mph by 2.237, knots by 1.944), then find the largest threshold the speed reaches or exceeds: 0.3, 1.6, 3.4, 5.5, 8.0, 10.8, 13.9, 17.2, 20.8, 24.5, 28.5 or 32.7 m/s. For example, 30 km/h ≈ 8.3 m/s, which exceeds 8.0 but not 10.8, so it is force 5, fresh breeze.
What Beaufort force is considered strong or dangerous wind?
Force 6 (10.8–13.8 m/s) is already uncomfortable — umbrellas become hard to use. Force 8 (gale, from 17.2 m/s) breaks twigs and impedes walking; force 10 (storm, from 24.5 m/s) uproots trees. Many countries issue wind warnings from force 8–9. Gusts can run 30–50% above the mean, so a force 8 mean can produce force 10 gusts — check gust forecasts before making safety decisions.
Why does the same wind speed give a different force in mph or knots than in m/s?
The official WMO thresholds are defined only in m/s; the km/h, mph and knot bands are rounded equivalents. If your speed sits very close to a boundary (e.g. 13.8 vs 13.9 m/s), entering it in a rounded unit can shift it just across the line and change the force by one. For borderline values, enter the speed in m/s for the most accurate result.
Does the Beaufort scale measure gusts?
No. The scale describes the mean wind, averaged over 10 minutes at a height of 10 metres. A single strong gust of 20 m/s does not mean force 9 — it may occur while the mean wind stays within force 8. For activities like sailing, cycling or working at height, always consider both the mean wind (Beaufort force) and the forecast gust speeds.
What is the difference between Beaufort force 12 and a hurricane category?
Beaufort 12 simply means a mean wind of 32.7 m/s or more — everything above that threshold is force 12. The Saffir–Simpson hurricane categories (1–5) subdivide that same range using a hurricane's maximum sustained 1-minute wind, a different averaging period measured at a different height. So Beaufort 12 covers everything from a minimal hurricane to a category 5.
Can I estimate the Beaufort force without an anemometer?
Yes — that is the scale's original purpose. Watch the effects around you: rustling leaves and a moving vane suggest force 2–3; dust and loose paper rising means force 4; large branches moving and umbrellas struggling indicate force 6; walking becomes difficult around force 7; twigs snapping off trees signal force 8. At sea, white horses appear from force 4–5 and foam streaks from force 7. The calculator's description field lists the full set of visual clues.
How do I convert knots to Beaufort force, as used in marine forecasts?
Marine forecasts often quote winds in knots, and each Beaufort force maps to a knot range: force 4 is 11–16 kn, force 6 is 22–27 kn, force 8 is 34–40 kn, force 10 is 48–55 kn. Simply select knots as the unit in the calculator, or remember the rough rule that a knot is about 0.51 m/s when checking thresholds by hand.