Beaufort Scale Converter

Introduction to the Beaufort wind force scale

The Beaufort scale is a thirteen-step index, running from force 0 to force 12, that describes wind strength by what the wind visibly does rather than by what an instrument reads. Force 0 is a mirror-flat sea and smoke that climbs straight up; force 12 is a sea completely white with driven spray and destruction ashore. Because every step is anchored to a written description of the sea surface and of the landscape, a Beaufort number can be reported from the deck of a boat with no anemometer at all, and it will still mean the same thing to a forecaster on the other side of the world. That is the property that kept the scale alive for two centuries after the sailing navy that produced it disappeared.

Modern practice pairs the descriptions with speed bands, and those bands are what this converter works with. Each force covers a range of wind speeds measured as a ten-minute mean at the standard exposure height of ten metres over open, level ground or open sea. Reporting a force therefore compresses a continuous measurement into a coarse but robust category, which is exactly what you want when the underlying observation is a judgement made by eye in poor visibility on a moving deck.

Where the scale came from

Commander Francis Beaufort of the Royal Navy set out his version of the scale in his private log in 1806, drawing on earlier wind notations by John Smeaton and Alexander Dalrymple. His steps were defined not by speed but by canvas: force 4 was the wind under which a well-conditioned man-of-war could carry royals in chase, while force 10 was the wind that would let her show only close-reefed main topsail and reefed foresail. The Admiralty made the scale standard for Royal Navy logs in 1838, and it was extended with the wave and land criteria that observers still use after steam replaced sail and the canvas definitions became meaningless.

The numerical speed equivalents were fitted later. The relation adopted internationally in 1946, and still used to generate the published tables, ties the force number to the mean wind speed by a simple power law, which is why the bands get wider as the force number climbs.

How to use the Beaufort scale converter

The tool runs in two directions. In Speed to Beaufort mode, choose the unit you actually measured in — knots, mph, km/h or m/s — enter the speed, and the converter looks the value up in the published band table for that unit. It returns the force number, its name, the land description, the sea state description, the probable open-sea wave height for that force, and the same band expressed in all four units. In Beaufort to speed mode you supply a force number from 0 to 12 and get the band limits back, which is what you need when a shipping forecast or an old logbook quotes a force and you want the speeds behind it.

Two details are worth knowing. First, the converter looks your value up in the table for the unit you entered, because that is how the official tables are meant to be read, and it warns you when converting the same wind into a different unit would land it in a neighbouring force. Second, the copy button puts a one-line summary on the clipboard, including the force, the name and the observation cue, so it can be pasted straight into a log entry or a message to a crew.

The Beaufort formula and where the band edges come from

The band edges are not arbitrary. The internationally adopted relation between the force number and the mean wind speed in metres per second is

Formula: v = 0.836 ⁢ B^3/2

and inverting it gives the estimate you can run in your head from a measured speed:

Formula: B ≈ (v/0.836)^2/3

The published band limits are the speeds that sit halfway between neighbouring forces, that is the values of the first expression evaluated at , then rounded to whole units. Force 7 and force 8, for example, are separated by m/s, which is 33.4 knots, so the tables put the boundary at 34 knots. The same arithmetic, converted with and rounded independently, produces the 62 km/h edge in the metric column. The converter stores the rounded published limits rather than recomputing the power law, so its answers match the tables that forecasters and mariners actually read.

The full Beaufort band table

This is the complete comparison table the converter uses. Wave heights are the probable heights for a fully developed sea in the open ocean; enclosed or shallow water produces shorter, steeper waves at the same wind speed.

Reading the wind on land

Ashore, the useful cues climb a ladder of stiffness. Smoke is the most sensitive indicator there is: it rises vertically only in a true calm, leans visibly in force 1 before a wind vane will even turn, and is shredded within a few metres of the chimney above force 6. Leaves take over next — a rustle is force 2, constant motion of twigs is force 3, and dust and loose paper lifting off the ground marks force 4. Whole small trees swaying puts you at force 5, large branches in motion with a whistle in overhead wires is force 6, and by force 7 you notice the wind in your own walking. Above that the cues become damage rather than motion: twigs snapping off at force 8, slates and chimney pots coming loose at force 9, uprooted trees at force 10.

Two cautions apply on land. Shelter from buildings, hedges and terrain can easily cost you two forces, so a sheltered garden will read calmer than the exposed ridge a kilometre away. And gusts routinely reach 1.4 to 1.6 times the mean speed, so the strongest thing you feel is not the number you should report.

Reading the sea state

At sea the scale reads more cleanly because the water integrates the wind over time and distance. The first breaking crests, the scattered white horses, appear at force 3. They become frequent at force 4 and numerous at force 5. Force 6 is when foam crests become extensive and spray starts to blow. The single most reliable transition is force 7: foam begins to be blown in streaks that line up along the wind direction, and once you see organised streaking rather than scattered patches you are in near-gale conditions. Force 8 adds spindrift torn off the crests, force 9 makes the streaks dense and starts to cut visibility, and by force 10 the whole surface looks white.

Sea state lags the wind, which is the main trap. A wind that has only just risen produces a sea one or two forces lower than its speed, while a sea left over from a passing gale looks worse than the wind now blowing. Fetch and depth matter too: a force 6 in a narrow loch will never build the three-metre waves the table quotes for open ocean.

Worked example: a 28 mph coastal wind

Suppose a handheld anemometer on an exposed headland averages 28 mph. The mph column places 28 in the 25–31 band, so this is Beaufort 6, a strong breeze. The converter reports the band in every unit: 22–27 knots, 25–31 mph, 39–49 km/h and 10.8–13.8 m/s, with a probable open-sea wave height near 3 metres. The land description tells you large branches will be moving and umbrellas will be unusable, and the sea description warns of extensive foam crests and blown spray — enough to keep a dinghy fleet ashore.

Now run it the other way. A shipping forecast announces a gale, force 8. Entering 8 in Beaufort to speed mode returns 34–40 knots, 39–46 mph, 62–74 km/h and 17.2–20.7 m/s, with a probable wave height of 5.5 metres. Since gusts commonly run about 1.4 times the mean, the same forecast implies gusts to roughly 50 knots, which is the number that actually decides whether a mooring holds. Finally, note the edge case the tool flags: a 16-knot wind converts to 29.6 km/h, which rounds to 30 km/h and lands in the force 5 row of the metric column even though the knot column calls 16 knots force 4. The wind has not changed; only the rounding has.

Limitations and assumptions behind a Beaufort estimate

The scale carries several assumptions that are easy to forget. It assumes a mean wind, so it says nothing about gusts, and structural damage is usually done by the gust rather than the mean. It assumes standard exposure at ten metres over open ground, so readings taken at head height, in a street canyon or on a hilltop are not directly comparable. It assumes a fully developed sea when you use the wave-height column, which requires both fetch and duration. And because each force spans a range, a Beaufort report deliberately discards precision: force 8 is any wind from 34 to 40 knots, a spread wide enough to matter to a small boat.

Above force 12 the scale simply stops. Extensions to force 17 were introduced for typhoon reporting in parts of Asia and are used by some services, but they are not part of the international standard, and elsewhere stronger winds are described with the Saffir–Simpson hurricane categories or reported directly in knots. Other limitations are practical: the land descriptions were written for temperate vegetation and traditional roofing, they translate poorly to desert or urban settings, and the sea descriptions assume an observer with a clear view of open water. Treat a Beaufort number as a robust category, not as a measurement, and quote the measured speed alongside it whenever you have one.

Frequently asked questions about the Beaufort scale

Related planning tools

Continue weather prep by estimating cold stress with the wind chill calculator, comparing structural forces using the wind load calculator, and evaluating severe-weather sheltering in the storm shelter cost-benefit tool.

Sources: the band limits, force names, land criteria, sea criteria and probable wave heights used by this converter are taken from the references below.

• WMO Manual on Codes, Volume I.1 (WMO-No. 306), Code table 1100 — Beaufort wind force scale
• UK Met Office — Beaufort wind force scale
• NOAA National Weather Service — Beaufort wind scale
• NOAA NCEI — International Comprehensive Ocean-Atmosphere Data Set documentation on Beaufort equivalent scales