Last updated: 2026-07-13

Wind Speed and Beaufort Scale

The Beaufort scale turns a wind speed into a single force number from 0 to 12, each paired with a plain description of what the wind does on land and at sea. This converter works in both directions: enter a wind speed in km/h, mph, knots, or m/s to get its Beaufort force, or enter a Beaufort number to see the typical wind speed it stands for.

Where the scale comes from

The scale was devised in 1805 by Francis Beaufort, an officer of the Royal Navy, to standardise how ships' logs recorded the weather. In its first form it described the wind by how much sail a full-rigged frigate could carry, from "calm" up to "that which no canvas could withstand". As instruments improved, the scale was tied to observable effects on land and sea, and eventually to measured wind speed. The World Meteorological Organization still defines each force by a range of the mean wind speed measured 10 metres above open ground.

Because a force number carries a description as well as a speed, it remains a compact way to communicate conditions. Marine and coastal forecasts issue gale warnings at force 8 and storm warnings at force 10 precisely because the associated sea state and hazard are widely understood.

The full Beaufort scale

The table below lists each force with its wind-speed range and the classic descriptions on land and at sea.

Force	Description	m/s	km/h	knots	mph	On land	At sea
0	Calm	< 0.5	< 1	< 1	< 1	Smoke rises vertically	Sea like a mirror
1	Light air	0.5–1.5	1–5	1–3	1–3	Smoke drifts, vanes still	Ripples without crests
2	Light breeze	1.6–3.3	6–11	4–6	4–7	Wind felt on face, leaves rustle	Small wavelets
3	Gentle breeze	3.4–5.4	12–19	7–10	8–12	Leaves and twigs in constant motion	Large wavelets, scattered whitecaps
4	Moderate breeze	5.5–7.9	20–28	11–16	13–18	Dust and loose paper lift, branches move	Small waves, frequent whitecaps
5	Fresh breeze	8.0–10.7	29–38	17–21	19–24	Small trees sway	Moderate waves, many whitecaps
6	Strong breeze	10.8–13.8	39–49	22–27	25–31	Large branches move, umbrellas hard to use	Large waves, white foam crests
7	Near gale	13.9–17.1	50–61	28–33	32–38	Whole trees in motion, walking is an effort	Sea heaps up, foam blown in streaks
8	Gale	17.2–20.7	62–74	34–40	39–46	Twigs break from trees, progress impeded	Moderately high waves, spindrift
9	Strong gale	20.8–24.4	75–88	41–47	47–54	Slight structural damage, slates lift	High waves, dense foam, rolling sea
10	Storm	24.5–28.4	89–102	48–55	55–63	Trees uprooted, considerable damage	Very high waves, sea appears white
11	Violent storm	28.5–32.6	103–117	56–63	64–72	Widespread damage, rarely inland	Exceptionally high waves
12	Hurricane force	≥ 32.7	≥ 118	≥ 64	≥ 73	Devastating destruction	Air filled with foam and spray, visibility poor

How the conversion works

Each force sits on a smooth relation between the wind speed and the number. With the mean wind speed $v$ in metres per second, the force is

$B = \operatorname{round}\!\left(\left(\frac{v}{0.836}\right)^{2/3}\right)$

and the result is held between 0 and 12. Running it the other way gives the mean wind speed at the centre of a force band:

$v = 0.836 \cdot B^{1.5}$

For a wind of 10 m/s:

$B = \operatorname{round}\!\left(\left(\frac{10}{0.836}\right)^{2/3}\right) = \operatorname{round}(5.23) = 5$

so 10 m/s is a fresh breeze, force 5. The same relation written for knots is $B = \operatorname{round}\!\left((v_{kn} / 1.625)^{2/3}\right)$, because one knot is about 0.514 m/s and $0.836 / 0.514 \approx 1.625$. Whichever unit you enter, the converter reduces it to metres per second first, so the force number is identical.

Reading the scale both ways

Going from a speed to a force is the common case: a forecast quotes 55 km/h and you want to know that this is a strong wind on the edge of a gale. Going the other way is useful when a marine forecast is issued in Beaufort numbers and you want the equivalent speed for planning — a "force 8 gale" means a mean wind of roughly 17 to 21 m/s, gusting higher. Because force 12 has no upper limit, any sustained wind at or above 32.7 m/s is reported as hurricane force; storms stronger than that are graded by separate systems such as the Saffir–Simpson hurricane categories.

Frequently Asked Questions (FAQ)

What is the Beaufort scale?

The Beaufort scale is a 0-to-12 measure of wind force introduced by Admiral Sir Francis Beaufort of the Royal Navy in 1805. It was originally defined by the effect of the wind on a sailing ship and later by observable conditions on land and sea, so a sailor or observer could estimate the force without an instrument. Force 0 is calm, force 6 is a strong breeze, force 8 is a gale, and force 12 is hurricane force. Modern meteorological services still use it in marine and coastal forecasts because the described conditions are easy to picture.

How do I convert knots to a Beaufort number?

The scale is anchored to the mean wind speed in knots. A convenient approximation is B = (v / 1.625)^(2/3) with v in knots, rounded to the nearest whole number. For example, 22 knots gives (22 / 1.625)^(2/3) ≈ 5.6, which rounds to force 6, a strong breeze. This converter uses the equivalent metric form B = (v / 0.836)^(2/3) with the speed in metres per second, so the result is the same whichever unit you enter.

What wind speed is hurricane force on the Beaufort scale?

Force 12, hurricane force, begins at a sustained wind speed of about 32.7 m/s, which is 64 knots, 118 km/h, or 73 mph. The Beaufort scale stops at 12, so any sustained wind at or above that threshold is reported as force 12 regardless of how much stronger it is. Tropical-cyclone intensity beyond that point is described with separate systems such as the Saffir–Simpson categories.

Why use the Beaufort scale instead of a plain wind speed?

A single force number bundles a range of wind speeds with a description of what that wind actually does, which is useful when no anemometer is available or when a forecast needs to be understood at a glance. Marine forecasts still issue gale (force 8) and storm (force 10) warnings by Beaufort number because the associated sea state and hazard are widely understood. For engineering or precise records a measured wind speed is preferred, and this tool lets you move between the two.