The Beaufort wind scale: what force 0 to 12 means

The Beaufort scale rates wind strength from force 0 (calm, under 1 knot) to force 12 (hurricane force, 64 knots or more), with each number tied to a range of average wind speeds and to effects you can see, such as moving branches or breaking waves. Force 5 is a fresh breeze of 17 to 21 knots; force 8 is a gale of 34 to 40 knots. The Beaufort scale converter turns any speed into its force number and back, and this guide explains the scale itself.

What the Beaufort scale is

Francis Beaufort, a Royal Navy officer and hydrographer, devised the scale in 1805 so that sailors could report wind consistently; before that, one officer's stiff breeze might be another's gentle one. It was originally described by its effect on a ship's sails. In 1916 the descriptions were changed to the state of the sea and extended to land. Anemometer readings were standardised against the force numbers in 1923, and the modern scale follows an empirical relationship: wind speed ≈ 0.836 × B^1.5 m/s, where B is the force number and the speed is measured 10 metres above the surface.

The scale from 0 to 12

These are the standard ranges in knots, with metres per second in brackets:

• 0 Calm: under 1 kn (0–0.2 m/s)
• 1 Light air: 1–3 kn (0.3–1.5 m/s)
• 2 Light breeze: 4–6 kn (1.6–3.3 m/s)
• 3 Gentle breeze: 7–10 kn (3.4–5.4 m/s)
• 4 Moderate breeze: 11–16 kn (5.5–7.9 m/s)
• 5 Fresh breeze: 17–21 kn (8.0–10.7 m/s)
• 6 Strong breeze: 22–27 kn (10.8–13.8 m/s)
• 7 Near gale: 28–33 kn (13.9–17.1 m/s)
• 8 Gale: 34–40 kn (17.2–20.7 m/s)
• 9 Strong gale: 41–47 kn (20.8–24.4 m/s)
• 10 Storm: 48–55 kn (24.5–28.4 m/s)
• 11 Violent storm: 56–63 kn (28.5–32.6 m/s)
• 12 Hurricane force: 64 kn or more (32.7 m/s or more)

A knot is one nautical mile per hour, about 1.15 mph or 1.852 km/h, so 20 knots is about 23 mph or 37 km/h, comfortably force 5. The ranges widen as the numbers rise because the speed grows with the force to the power 1.5. If you work across unit systems a lot, the unit converter handles speed alongside everything else, and Metric vs imperial: how we got two systems explains why the units differ.

Judging wind without instruments

The scale's lasting strength is that you can estimate it by looking around. On land: at force 0 smoke rises straight up; at force 2 you feel wind on your face and leaves rustle; force 4 raises dust and loose paper and moves small branches; at force 6 large branches move and umbrellas become hard to use; at force 7 whole trees sway and walking into the wind is an effort; at force 8 twigs break off trees. At sea the cues are the waves: small wavelets at force 2, frequent white horses at force 4, white foam crests and spray at force 6, and foam blown in streaks from force 7 upwards. I put the land and sea descriptions next to each speed in the converter for exactly this reason: they are how most people actually experience wind.

Beaufort in forecasts and warnings

Most weather forecasts now give wind in mph, km/h or knots, but the Beaufort scale survives where consistency matters. The UK Met Office shipping forecast broadcast on BBC Radio 4 still reports winds as force numbers, such as "southwest 6 to gale 8". Warning systems are built on it too: in the United States, force 8 or 9 brings a gale warning, force 10 or 11 a storm warning and force 12 a hurricane-force wind warning. Note that "storm" and "hurricane" here describe wind strength only; a force 12 wind does not mean a tropical cyclone is present.

Gusts, waves and limits

The Beaufort force describes the sustained average wind, not the peak gusts, which can be considerably stronger than the average a forecast quotes. The wave heights often printed with the scale (about 3 m at force 6, 5.5 m at force 8) are the Met Office's typical values for well-developed waves in the open sea; near the coast, or soon after the wind rises, the sea will be lower. Wind also feels colder than the thermometer suggests, which the feels-like temperature calculator works out from wind chill. Finally, speeds converted between units are rounded at the boundaries, so a reading that sits exactly on the edge between two forces could be reported either way.