Beaufort Wind Scale

Describe a wind by its force on the Beaufort scale rather than by a raw speed. Enter a wind speed in any of four units, and the page converts it, assigns the matching force from zero to twelve, and prints the conditions that force produces on land and at sea.

Reading Wind as a Force, Not a Number

The Beaufort scale predates the anemometer as a practical instrument, and it was written to be read by eye rather than measured. This page takes a numeric speed, normalises it to a single unit, and returns the force band it falls into along with the description that band carries.

• Four input units are accepted: miles per hour, kilometres per hour, metres per second and knots. Whichever you choose is converted to miles per hour before the lookup happens.
• The scale runs from force 0 to force 12, and each band is defined by an upper bound that is exclusive, so a speed only enters the next force once it has passed the boundary rather than reached it.
• Each force carries a short name, such as calm, light breeze or gale, and a one-line description of what that wind does to trees, dust, chimneys or the surface of the sea.
• The boundaries are spaced unevenly: the early bands cover one or a few miles per hour each, while the top bands span ten or more, which mirrors how much more destructive a small proportional increase becomes at high wind.
• The result prints the value you entered in its original unit and, beside it, the same speed converted to miles per hour, so you can see the figure the lookup actually used.
• A reference table listing every band and its range is appended to the result for context.

Getting a Force Reading

1. Enter the wind speed as a number. The field accepts decimals, so a reading such as 24.9 is valid.
2. Choose the unit the number is in: miles per hour, kilometres per hour, metres per second or knots.
3. Press the convert button to run the lookup.
4. Read the large force number and its name at the top of the result.
5. Check the speed line beneath it, which repeats your input and shows the equivalent in miles per hour.
6. Read the description next, and use it to judge whether the force matches what you can actually see outside.
7. Change the speed or the unit and convert again; each run replaces the previous result.

Where Speeds Land on the Scale

The boundaries are exclusive, so the exact value at a threshold belongs to the higher band. These readings show how that plays out, including the default of 25 miles per hour.

• At the default of 25 miles per hour the lookup returns force 6, named strong breeze, with large branches moving and a whistling in overhead wires.
• The same reading a tenth of a mile per hour lower, at 24.9, returns force 5, named fresh breeze, because the force 6 band begins strictly above 25 and does not include it.
• A speed of 12.9 miles per hour stays at force 3, while 13 crosses into force 4, showing the same exclusive-boundary rule lower down the scale.
• Entering 47 knots converts to about 54.1 miles per hour and lands on force 9, a strong gale, where structural damage becomes possible.
• Entering 40 kilometres per hour converts to about 24.9 miles per hour and therefore returns force 5, one band below the default, even though 40 sounds like a larger number at first glance.
• At the top of the scale, 83 miles per hour returns force 11, a violent storm, and 84 crosses into force 12, the hurricane band.

Different Ways to Express Wind Strength

Alternative	Notes
Descriptive force	The Beaufort approach, which names what the wind is doing rather than how fast it moves. It is usable without an instrument and travels well in conversation, but it is coarse and depends on an honest observer.
Measured speed	An anemometer gives a number in a chosen unit, which is precise and recordable but says nothing on its own about what the wind feels like or what it will damage.
Named storm categories	At the top end, separate category systems classify tropical systems by sustained wind, adding information about pressure and structure. Those systems begin where the everyday Beaufort scale is already at its last band.
Practical effect	For a decision such as whether to go out on the water or secure a loose roof tile, the observable effect matters more than the number. The descriptions attached to each force are the part meant to be used directly.

A Lookup Table, Not a Forecast

The conversion is a piece of arithmetic and a fixed set of bands held in the page. It contains no weather data and asks for none, so it can only classify a wind speed you already have.

• No network request is made; the unit conversion and the band lookup both run in the browser.
• The scale itself is a fixed reference, so the result for a given speed and unit never changes between visits.
• Only the magnitude of the wind is used. Direction, gustiness, temperature and duration are not part of the scale and are not modelled here.
• Submitting a negative speed is treated as invalid, and a non-numeric entry produces a prompt to enter a valid value instead of a force.
• The appended reference table is static text, not a computed summary, so it describes the full scale regardless of which force your reading returned.
• The page stores nothing, and each conversion overwrites the last.

Wind Scale Questions

Why does exactly 25 mph give force 6 rather than force 5?

Because the bands are defined with strict upper limits. Force 5 covers speeds below 25, so a reading of 25 has already passed that ceiling and falls into the next band, which runs up to but not including 32. The reference table prints the bands as whole-number ranges, which makes the boundary look inclusive when the calculation is not.

Which unit does the scale actually work in?

Internally the page converts everything to miles per hour and then looks the speed up in that unit. Kilometres per hour are multiplied by about 0.621, metres per second by about 2.237 and knots by about 1.151 to reach the same basis, so the chosen unit changes only the number you type, not the band that results from it.

What does the second speed on the result line mean?

It is your input converted to miles per hour, shown beside the original value and unit. The original figure is reproduced unchanged, and the conversion is printed alongside it so the speed the lookup used is visible. The pair is useful when the unit you entered is not the one you normally think in.

Does the scale stop at force 12?

In this page it does. Every speed from 84 miles per hour upward is grouped into force 12, the hurricane band, with no further subdivision. Some extended versions of the scale continue past twelve for exceptional storms, but those additional bands are not represented here.

Can I use the force number to judge whether it is safe to go out?

It gives a useful indication, since each force describes what the wind does to trees, dust and the sea, but it is a general guide rather than a safety ruling. Local exposure, gusts, direction and what you are doing all matter, and a sheltered spot can feel several bands calmer than an open one at the same measured speed.

Last verified: October 3, 2026

Launch Beaufort Wind Scale Converter

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Maintained by Huang Bin · Last updated October 3, 2026