Beaufort Scale Converter
Turn wind speed in km/h, mph, m/s or knots into a Beaufort number and description.
Quick answer
- Formula: Convert speed to m/s, then match WMO bands: <0.5 = 0 Calm, 0.5–1.5 = 1 Light air, … ≥32.7 = 12 Hurricane
- Worked example — Beaufort number: 4 (full example below).
Beaufort Scale Converter
| Beaufort number | 4 |
| Description | Moderate breeze |
| Wind speed | 5.56 m/s |
| Summary | 20 km/h = Beaufort 4 — Moderate breeze |
20 km/h is 5.56 m/s — Beaufort 4, a Moderate breeze: dust and loose paper blow, small branches move.
The Beaufort scale describes wind by its effects, from 0 (Calm) to 12 (Hurricane), using the World Meteorological Organization thresholds in metres per second: force 4 (Moderate breeze) starts at 5.5 m/s, force 7 (Near gale) at 13.9 m/s, and hurricane force at 32.7 m/s.
Enter wind speed in km/h, mph, m/s or knots and get the Beaufort number, its standard name, and the speed in m/s. Invented in 1805 for naval use, the scale is still how marine forecasts describe wind.
The formula
Worked example
20 km/h is 5.56 m/s — Beaufort 4, a Moderate breeze: dust and loose paper blow, small branches move.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
What wind speed is a gale?
Beaufort 8, from 17.2 m/s (62 km/h, 39 mph) up to 20.7 m/s. Beaufort 7 is a 'Near gale' and 9 a 'Strong gale'.
How strong is hurricane force on the Beaufort scale?
Beaufort 12 starts at 32.7 m/s — 118 km/h or 73 mph, the same threshold as a Category 1 hurricane on the Saffir–Simpson scale.
Why do marine forecasts say 'force 6'?
The Beaufort number is a compact, language-independent shorthand. Force 6 is a Strong breeze (10.8–13.8 m/s): large branches move and umbrellas become hard to use.
Category: Unit Conversions calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
Embed this calculator on your site
Free to use with attribution — paste the snippet below and the calculator appears on your page, with a link back to CalcHub.