Updated November 2026 · free, works in your browser
Formula and sources
The Beaufort number B relates to wind speed v at 10 m height by v = 0.836 × B^(3/2) m/s. Ranges in this tool follow the standard Beaufort chart. Conversions: 1 knot = 1.852 km/h ≈ 1.1508 mph ≈ 0.5144 m/s. In speed mode the speed is converted to knots and matched to the force whose range it falls in.
The Beaufort scale turns a wind force number from 0 to 12 into a wind speed and a description of the sea. Force 5, a fresh breeze, means 17 to 21 knots, which is 29 to 38 km/h. Enter a force or a speed above and the matching values appear instantly.
How to use the converter
- Pick "Force → speed" if you have a Beaufort number from a forecast, or "Speed → force" if you have a wind speed.
- In force mode, type a whole number from 0 to 12.
- In speed mode, type the wind speed and choose knots, km/h, mph or m/s.
- Read the force, its name, the speed ranges in four units and the typical sea state.
- Use the copy button to share a link to the exact result.
How it is calculated
The scale was devised by Francis Beaufort in the early nineteenth century and is now defined by wind speed bands, as in the standard WMO chart. The empirical relation is v = 0.836 × B^(3/2), where B is the Beaufort number and v is the wind speed in metres per second at 10 metres above the surface.
This tool uses the standard chart ranges rather than the rounded formula. For a speed input, the value is first converted to knots (1 knot = 1.852 km/h, 1 mph = 0.868976 knots, 1 m/s = 1.943844 knots) and then matched to the force whose lower limit it has reached.
Beaufort scale chart
| Force | Name | Knots | km/h | mph |
|---|---|---|---|---|
| 0 | Calm | 0–1 | 0–1 | 0–1 |
| 1 | Light air | 1–3 | 1–5 | 1–3 |
| 2 | Light breeze | 4–6 | 6–11 | 4–7 |
| 3 | Gentle breeze | 7–10 | 12–19 | 8–12 |
| 4 | Moderate breeze | 11–16 | 20–28 | 13–18 |
| 5 | Fresh breeze | 17–21 | 29–38 | 19–24 |
| 6 | Strong breeze | 22–27 | 39–49 | 25–31 |
| 7 | Near gale | 28–33 | 50–61 | 32–38 |
| 8 | Gale | 34–40 | 62–74 | 39–46 |
| 9 | Strong gale | 41–47 | 75–88 | 47–54 |
| 10 | Storm | 48–55 | 89–102 | 55–63 |
| 11 | Violent storm | 56–63 | 103–117 | 64–72 |
| 12 | Hurricane force | 64 or more | 118 or more | 73 or more |
Frequently asked questions
How many knots is Beaufort 6?
Force 6, a strong breeze, covers 22 to 27 knots, or 39 to 49 km/h. Large waves form and whitecaps are widespread.
What is the difference between a knot and km/h?
A knot is one nautical mile per hour. Since a nautical mile is 1.852 km, one knot equals 1.852 km/h. Multiply knots by about 1.85 to get km/h.
Is the Beaufort scale about gusts or average wind?
It describes a mean wind speed over a period of time. Gusts can be noticeably stronger than the force number suggests, so forecasts often give them separately.
Why is there a gap between some ranges, such as 3 and 4 knots?
The chart lists whole numbers, so ranges seem to skip a value. For decimals, this tool places the speed in the force whose lower limit has been reached, so 3.5 knots counts as force 1.
Does the sea state always match the force?
Not exactly. The sea descriptions are for open water with a steady wind. Fetch, currents, tide and swell from distant weather can make real waves higher or lower, and sheltered water is usually calmer.
From wind forecast to travel plans
Knowing what a force number means helps you read a marine forecast before a boat trip, ferry crossing or charter week. Wind and sea conditions affect departure times and comfort on board, so they also shape where and when you stay on the coast. Checking the forecast early helps you choose sensible dates and a base near the harbour or marina.
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