Beaufort Scale Table
| Force | Knots | mph | The land sign |
|---|---|---|---|
| 0 Calm | <1 | <1 | smoke rises vertically |
| 1 Light air | 1-3 | 1-3 | smoke drifts, vanes still |
| 2 Light breeze | 4-6 | 4-7 | leaves rustle, wind felt |
| 3 Gentle breeze | 7-10 | 8-12 | twigs in constant motion |
| 4 Moderate breeze | 11-16 | 13-18 | dust and loose paper lift |
| 5 Fresh breeze | 17-21 | 19-24 | small trees sway |
| 6 Strong breeze | 22-27 | 25-31 | large branches move - advisory line |
| 7 Near gale | 28-33 | 32-38 | whole trees sway, walking labors |
| 8 Gale | 34-40 | 39-46 | twigs break off trees |
| 9 Strong gale | 41-47 | 47-54 | slight structural damage |
| 10 Storm | 48-55 | 55-63 | trees uprooted, damage |
| 11 Violent storm | 56-63 | 64-72 | widespread damage |
| 12 Hurricane | 64+ | 73+ | devastation |
The Beaufort scale turns a wind speed into a force - thirteen steps from 0 (smoke rises vertically, sea like a mirror) to 12 (air filled with foam and spray), each defined by what the wind visibly does to trees, dust and waves. Type the wind speed in knots or miles per hour and the converter reads back the force, the name, and the speed band it sits in.
Admiral Sir Francis Beaufort standardized the ladder for the Royal Navy in 1805 so a sailing master could log wind without instruments, and it stuck: weather services still issue it, forecasts in maritime English still speak in forces, and the table below carries the land and sea signs a look-out checks. The boundary that matters most is force 6 - small-craft advisories begin where strong breeze ends.
How to use
- Type the observed wind speed in knots or mph - the force, its name and the full speed band appear instantly.
- Cross-check the sign, not just the number: force 4 is where dust and loose paper lift, force 6 is where large branches move and small-craft flags go up.
- The scale is a lookup, not an average: winds sit inside a band, and 33 knots is still force 7 until the 34th knot turns the gale.
Frequently asked questions
What wind speed is Beaufort force 6?
Twenty-two to twenty-seven knots - twenty-five to thirty-one mph - named strong breeze: large branches in constant motion, umbrellas hard to hold, wires whistle. At sea it brings large waves, extending white foam crests and spray. The number carries weight because small-craft advisories key off roughly this band; it is the traditional go-or-stay-out line for working boats.
Why is the scale in forces instead of exact speeds?
Because 1805 sailing masters had no anemometers - they had eyes. Beaufort defined each force by what canvas a full-rigged warship could carry and what the sea surface showed, which made the estimate repeatable between officers. The knot ranges were fitted to the observations afterward, so the scale is calibrated to visible evidence first and measured speeds second, which is exactly why it still works when instruments fail.
What force is a hurricane?
Force 12 - the top and original ceiling of the scale, sixty-four knots and up, where the air fills with foam and spray and the sea is completely white with driving visibility near zero. Beaufort's ladder stopped at 12 because nothing above it could be distinguished from a ship's log; the extended 13-to-17 scale exists in some national tables for over-land tornado-class winds but is not part of the maritime standard.
Do weather services still use the Beaufort scale?
Yes - the National Weather Service publishes the table and uses force terminology in marine forecasts; many national services describe coastal and small-craft conditions by Beaufort force rather than raw speed, and lake and sea state codes in shipping forecasts descend from it. For sailors it remains the shared language: 'force 6, occasional 7' says more to a crew than a decimal knots figure.
How does the scale relate to the names gale and storm?
They are forces, not adjectives: gale is force 8 (thirty-four to forty knots, twigs break, foam blows in streaks), strong gale 9, storm 10 (forty-eight knots and up, trees uproot, building damage begins), violent storm 11. A forecast saying 'gale 8 to storm 10' is handing you the whole deterioration in two words - each step is a visible change ashore, not a rounding.