Beaufort Wind Scale Calculator

How the Beaufort Scale and 1946 Formula Work

The Beaufort scale ranks wind from force 0 (calm) to force 12 (hurricane). On its Beaufort Wind Scale page, the National Weather Service Miami explains: "One of the first scales to estimate wind speeds and the effects was created by Britain's Admiral Sir Francis Beaufort (1774-1857). He developed the scale in 1805 to help sailors estimate the winds via visual observations. The scale starts with 0 and goes to a force of 12." In 1946, weather services tied each force to a 10 m wind speed. As the Icelandic Meteorological Office states: "The equation: W = 0.836B 3/2 where W is windspeed in m/s and B is value on the Beaufort-scale."

This page runs that 1946 equation both ways. When you enter a speed, it converts your value to m/s using the exact factors in NIST Special Publication 811 Appendix B.8. Next, it finds B = (v / 0.836)^(2/3) and rounds B to the nearest whole number up to 12. The band for force n runs from 0.836 × (n − 0.5)^1.5 to 0.836 × (n + 0.5)^1.5 m/s. When you pick a force number instead, the page computes the center speed v = 0.836 × B^1.5 m/s. It then shows that speed in mph, knots, and km/h. You can also convert any speed directly in the speed converter.

Printed agency tables list whole-number bands rather than decimals. The table above shows the NWS Miami mph and knots bands beside the 1946 formula bands. Because 0.836 × B^1.5 sits slightly below a few rounded table edges, six whole-number speeds land one force higher in the formula than in the NWS table. Those six edge speeds are 18 mph, 31 mph, 3 knots, 16 knots, 21 knots, and 27 knots. Also, the NWS Miami table prints 72 mph under both force 11 (64-72 mph) and force 12 (72-83 mph). By contrast, NWS Tampa Bay prints 64-73 mph for force 11 and 74-95 mph for force 12.

The scale is an observation table, not a weather warning. The Icelandic Meteorological Office notes: "This empirical relation between windforce, estimated on the Beaufort-scale, and the windspeed is based on the 10 minute average of the windspeed. In reality, windspeeds can vary a lot in that time. As a result, windgusts can not be measured with the Beaufort-scale." To see how wind shapes winter chill or summer heat, pair your 10 m speed with the wind chill calculator, the heat index calculator, or the dew point calculator.

The 1946 Beaufort Formula & Unit Factors

v (m/s) = 0.836 × B^(3/2)        B = (v / 0.836)^(2/3)
force = min(12, floor(B + 0.5))  (nearest whole number, capped at 12)
band of force n: 0.836 × (n − 0.5)^1.5 to 0.836 × (n + 0.5)^1.5 m/s
1 mph = 0.44704 m/s;  1 knot = 1852/3600 m/s;  1 km/h = 1/3.6 m/s
Dynamic wind pressure at sea level: q (Pa) = 0.5 × 1.225 × v(m/s)^2 = 0.42807 × B^3

Worked Beaufort Examples (20 mph, 15 m/s & Force 8)

Take the default wind speed of 20 mph. Multiplying by 0.44704 gives v = 8.94 m/s (17.4 knots, 32.2 km/h). The 1946 equation gives B = (8.9408 / 0.836)^(2/3) = 4.85 (4.854). That rounds to force 5 (fresh breeze). Force 5 spans 17.9 to 24.1 mph around a center speed of 20.9 mph. It puts 49.0 Pa of wind pressure on a surface at sea level. The whole-number NWS table also lists 20 mph under force 5 (19 to 24 mph).

Now take a mean wind of 15 m/s (33.6 mph, 29.2 knots, 54.0 km/h). Dividing by 0.836 and raising to the 2/3 power gives B = (15 / 0.836)^(2/3) = 6.85 (6.854). That rounds to force 7 (near gale, 137.8 Pa). Going from force to speed, force 6 gives 0.836 × 6^1.5 = 12.29 m/s (27.5 mph). Force 10 gives 0.836 × 10^1.5 = 26.44 m/s (59.1 mph, 51.4 knots), and force 12 gives 34.75 m/s (77.7 mph).

Scope, Table Edges & Anemometer Assumptions

• Wind speed is a 10-minute mean at standard 10 m height over open ground or water. Short 3-second gusts, sheltered yards, and other sensor heights are not modeled directly.
• The 1946 fit v = 0.836 × B^1.5 m/s differs by one force from the whole-number NWS table at six edge speeds. Those six speeds are 18 mph, 31 mph, 3 knots, 16 knots, 21 knots, and 27 knots.
• Force 9 is named Strong gale (severe gale in the NWS table). As the UK Met Office explains: "The official term is Strong gale, however, the Met Office uses the descriptive term Severe gale." Also, the NWS Miami table prints 72 mph under both force 11 and force 12. This page keeps that printed table and notes the overlap.
• Speeds above 32.6 m/s (72.9 mph, B >= 11.5) are capped at force 12. The page's top speed limit is 120 m/s (268 mph), and wind pressure uses standard sea-level air density (1.225 kg/m³).

What to Do With a Wind Speed or Beaufort Number

Match the force number to outdoor clues on land. On the NWS Miami Beaufort Wind Scale table, force 4 (13 to 18 mph) "Raises dust and loose paper; small branches are moved." At force 6 (25 to 31 mph), "Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty." At force 7 (32 to 38 mph), "Whole trees in motion; inconvenience felt when walking against the wind."

Separate mean speed from gusts before you convert. A forecast with a 15 mph mean wind and a 25 mph gust is force 4 on the mean, not force 6. The Beaufort scale is built on a 10-minute average speed.

Tie down garden stakes, row covers, and patio umbrellas before force 6 (25 mph, 76.5 Pa). At force 5 (19 to 24 mph), small trees in leaf sway. By force 8 (39 to 46 mph, 219.2 Pa), twigs snap off trees and walking is hard.

For boating or coastal storm checks above force 6 (22 knots, 3.0 m waves), read official National Weather Service advisories. Gale Warnings cover 34 to 47 knots (forces 8 and 9), and Storm Warnings start at 48 knots (force 10).

Beaufort Wind Scale Questions