Beaufort Wind Calculator
This Beaufort wind calculator converts a measured wind speed into a Beaufort force number, the 0 to 12 scale that mariners and meteorologists use to describe wind by its observable effects. The scale was devised so each step matches a recognizable change at sea, from a glassy calm at force 0 to a hurricane-force wind at force 12. The US National Oceanic and Atmospheric Administration publishes the modern equivalence between the force numbers and measured wind speed. This calculator uses the standard empirical relation that recovers the scale from speed: the force number is proportional to the wind speed in knots raised to the two-thirds power. Enter a sustained wind speed in knots and it returns the continuous force value, the nearest whole Beaufort number, and the descriptive name for that force. Use it to log conditions, interpret a marine forecast, or teach the scale. Because the relation is continuous, it interpolates between the official speed bands while still rounding back to the printed whole-number force. Every figure is computed deterministically from the published Beaufort relation shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself, and a reference table of the full scale.
The Beaufort relation maps speed to force: a wind of 20 knots gives a continuous force of 5.33, which rounds to Beaufort force 5, a fresh breeze. Each force step corresponds to a recognizable change in the sea state.
Beaufort wind formula
B = ( v / 1.625 ) ^ (2 / 3)
v = wind speed in knots
B = Beaufort force (round to nearest whole number)
The relation is the inverse of the empirical curve that defines the scale, where wind speed grows as the force number raised to the power 1.5. Solving for the force gives the two-thirds power law used here, with the constant 1.625 set so the bands match the official speed ranges.
Worked example
Convert a sustained wind of 20 knots into a Beaufort force number.
- v / 1.625 = 20 / 1.625 = 12.307692
- B = 12.307692 ^ (2 / 3) = 5.33
- round 5.33 to the nearest whole number: Beaufort force 5
- force 5 (17 to 21 knots) is described as a fresh breeze
A 20 knot wind gives a continuous value of 5.33, rounding to Beaufort force 5, a fresh breeze. These are the calculator's default inputs, so the result above matches the widget exactly.
The Beaufort wind scale
Each force number corresponds to a band of wind speed and a descriptive name.
| Force | Speed (knots) | Description |
|---|---|---|
| 0 | under 1 | Calm |
| 3 | 7 to 10 | Gentle breeze |
| 5 | 17 to 21 | Fresh breeze |
| 8 | 34 to 40 | Gale |
| 10 | 48 to 55 | Storm |
| 12 | 64 and above | Hurricane force |
Beaufort scale and wind reference: US National Oceanic and Atmospheric Administration (NOAA).
Beaufort Wind Calculator: frequently asked questions
What units does the calculator expect?
Enter the sustained wind speed in knots, the unit the marine Beaufort scale is defined in. One knot is one nautical mile per hour, about 1.15 statute miles per hour or 0.514 meters per second. If your forecast is in miles per hour or kilometers per hour, convert to knots first, or the force number will be off.
Should I use sustained speed or gusts?
Use the sustained wind speed, conventionally a ten-minute average. The Beaufort scale describes the steady wind and its typical effects on the sea, not brief gusts. Gusts can be 30 to 50 percent stronger than the sustained speed and are reported separately in forecasts.
Why a two-thirds power instead of a simple table?
The original scale is a set of speed bands, but a continuous formula is handy for interpolation and computation. The empirical curve that fits the bands has wind speed proportional to the force to the power 1.5, so inverting it gives force proportional to speed to the power two-thirds. The result rounds back to the same whole-number bands.
What does each force feel like?
Force 0 is calm with smoke rising vertically; force 3 is a gentle breeze that extends a light flag; force 5 is a fresh breeze raising moderate waves with many whitecaps; force 8 is a gale that breaks twigs off trees and impedes walking; force 12 is hurricane force with the air filled with foam and spray and visibility severely reduced.
Does the scale go above force 12?
The classic Beaufort scale tops out at force 12 for hurricane-force winds of 64 knots or more. Some national services extended it to force 17 for tropical cyclones, but the standard marine scale used in most forecasts stops at 12, with stronger winds quantified directly in knots or by hurricane category.
Official sources
- Beaufort scale and marine wind reference: US National Oceanic and Atmospheric Administration (NOAA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.