Beaufort Scale Calculator
Beaufort force 0 to 12 from wind speed, with effects.
What it does
The Beaufort scale grades wind by eye from 0 calm to 12 hurricane, tying each force to speeds and visible effects. Thirty miles per hour lands Force 6, Strong breeze, where large branches sway.
The tool above takes wind speed in mph or knots. It returns force number with name and land effects beside it. Knots convert at 1.15078 before banding.
Bands follow National Weather Service tables: 25 to 31 mph for Force 6, 8 to 12 for Force 3, 47 to 54 for Force 9, past 73 for Force 12. Sea states ride alongside, from mirror calm to completely white.
Admiral Sir Francis Beaufort built the scale in 1805 for sailors judging sail by observation. Modern instruments calibrate it, but the force numbers still speak effects first.
How to use
- Type the wind speed.
- Pick mph or knots.
- Read force first, name beside it.
- Read land effects for impact.
- Compare gusts separately; bands use sustained.
- Pair with warnings past Force 8.
Formula
- force
- Beaufort number 0 to 12.
- 1.15078
- mph per knot.
- 74+
- Force 12 hurricane threshold in mph.
Bands use sustained speeds; gusts grade separately. Force 9 reads Strong gale in WMO wording.
Worked example
Example: 30 mph
Defaults: 30 mph.
- Band: 30 sits in 25 to 31.
- Force: 6, Strong breeze.
- Effects: large branches sway.
- Knots check: 30 / 1.15078 is 26.1 knots, inside 22 to 27.
- Tool shows Force 6, Strong breeze, effects.
Reference table
| Force | mph | Name |
|---|---|---|
| 0 | 0 | Calm |
| 1 | 1 to 3 | Light air |
| 2 | 4 to 7 | Light breeze |
| 3 | 8 to 12 | Gentle breeze |
| 4 | 13 to 18 | Moderate breeze |
| 5 | 19 to 24 | Fresh breeze |
| 6 | 25 to 31 | Strong breeze |
| 7 | 32 to 38 | Near gale |
| 8 | 39 to 46 | Gale |
| 9 | 47 to 54 | Strong gale |
| 10 | 55 to 63 | Storm |
| 11 | 64 to 73 | Violent storm |
| 12 | 74+ | Hurricane |
Limits
- Sustained winds; gusts separate.
- Effects vary with terrain and trees.
- Marine states need sea tables.
- Local warnings override bands.
Common mistakes
- Grading gusts as sustained force.
- Mixing knots with mph bands.
- Reading Force 12 inside 74 mph cap.
- Expecting exact damage at boundaries.
- Skipping warnings for high forces.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 mph?
Force 3, Gentle breeze.
50 mph?
Force 9, Strong gale.
20 knots?
23 mph, Force 5, Fresh breeze.
Questions
30 mph force?
Force 6, Strong breeze.
Who made it?
Admiral Beaufort in 1805 for sailors.
Top force?
12 hurricane past 73 mph.
Knots input?
Yes: 20 knots converts to 23 mph, Force 5.
Gale force?
Force 8 at 39 to 46 mph.
Speed page versus this?
That converts units; this grades force and effects.