The Beaufort scale turns a number from 0 to 12 into a precise picture of the wind and the sea it raises. This reference lets you go either way: enter a measured wind speed to get the force, or scan the full table to read across force number, speed in three units, and probable wave height.
How it works
Each Beaufort force is defined by a wind-speed range in knots, which this tool converts to metres per second and kilometres per hour using 1 knot = 0.514444 m/s = 1.852 km/h. To turn a measured speed back into a force it uses the standard empirical relation:
v = 0.836 × B^1.5 (v in m/s, B = Beaufort number)
B = (v / 0.836)^(2/3) (rounded to the nearest whole force)The probable wave heights are the WMO open-sea values, describing a fully developed sea far from land.
Worked example
A measured 25-knot wind is about 12.9 m/s, which the formula places at Beaufort force 6, a strong breeze with large waves and extensive foam crests. Remember that the wave-height column assumes deep, open water with unlimited fetch: in a harbour, a river mouth, or with an offshore wind, the sea will be far smoother than the table suggests for the same force. Always treat the sea-state description as the most reliable field cue when no anemometer is available.
Understanding the scale in practice
The Beaufort scale was devised for practical observation before instruments were ubiquitous. A mariner or coastguard officer could look at the sea surface and rigging and arrive at the force with reasonable accuracy. Today it remains useful because it connects a number directly to observable effects — you do not need an anemometer to estimate whether you are in a strong breeze or a near gale.
Land observations alongside sea observations
The WMO scale also includes land-based descriptions for each force: force 3 (gentle breeze) keeps leaves and twigs in constant motion; force 7 (near gale) makes walking difficult and sways large trees noticeably; force 10 (storm) uproots trees and causes structural damage. When offshore data is unavailable, land observations can give a usable estimate of the wind strength over nearby water, though the sea state will still depend on fetch and fetch duration.
Warnings and what each force means operationally
| Force | Name | Operational meaning |
|---|---|---|
| 0–3 | Calm to gentle breeze | Suitable for all small craft |
| 4–5 | Moderate to fresh breeze | Small open boats should exercise caution |
| 6–7 | Strong breeze / near gale | Restrict open-boat operation |
| 8–9 | Gale / strong gale | Gale warning issued; small craft in harbour |
| 10–11 | Storm / violent storm | Major damage to structures ashore |
| 12 | Hurricane force | Catastrophic; measured wind 64 kn+ |
Harbour authorities and coast guards issue small-craft advisories from force 6 and formal gale warnings from force 8. The scale’s upper end (10–12) is rarely experienced inland and represents genuinely dangerous conditions even for large vessels.
Fetch and why wave height varies
The table’s wave heights assume unlimited fetch — open ocean with nothing to stop the waves building. Fetch is the unobstructed distance over which wind acts on the water surface. A force 7 gale blowing from land across a sheltered bay produces tiny waves; the same force 7 after 500 miles of ocean fetch produces the 4–5.5 metre waves the table predicts. This distinction is crucial for passage planning: arriving at a headland from a sheltered anchorage into the open sea can mean encountering dramatically higher seas than the current wind speed suggests if the wind has been blowing offshore for many hours.