Get trained
This page explains the principles. It is not a substitute for hands-on instruction from a recognized program such as US Sailing, the RYA or ASA, and it does not replace the safety rules that apply where you sail.
How it works
The calculator converts your speed to knots, rounds to the nearest whole knot and finds the force whose range contains it, using the Met Office's published ranges. It uses the Met Office's own conversions (knots times 1.15 for mph, knots times 0.514 for metres per second) and 1.852 km/h per knot. Wave heights are the Met Office's figures, which it says refer to well-developed wind waves of the open sea, and it warns that the sea lags behind the wind when the wind gets up. Sheltered water, a short fetch or a fresh breeze that has only just arrived can look very different.
The full scale
| Force | Wind term | Knots | Probable wave height (m) | Sea state |
|---|---|---|---|---|
| 0 | Calm | under 1 | none | 0, calm (glassy) |
| 1 | Light air | 1 to 3 | 0.1 | 1, calm (rippled) |
| 2 | Light breeze | 4 to 6 | 0.2 | 2, smooth |
| 3 | Gentle breeze | 7 to 10 | 0.6 | 3, slight |
| 4 | Moderate breeze | 11 to 16 | 1.0 | 3 to 4, slight to moderate |
| 5 | Fresh breeze | 17 to 21 | 2.0 | 4, moderate |
| 6 | Strong breeze | 22 to 27 | 3.0 | 5, rough |
| 7 | Near gale | 28 to 33 | 4.0 | 5 to 6, rough to very rough |
| 8 | Gale | 34 to 40 | 5.5 | 6 to 7, very rough to high |
| 9 | Severe gale | 41 to 47 | 7.0 | 7, high |
| 10 | Storm | 48 to 55 | 9.0 | 8, very high |
| 11 | Violent storm | 56 to 63 | 11.5 | 8, very high |
| 12 | Hurricane | 64 and above | 14 or more | 9, phenomenal |
The Met Office table gives strong gale as the official term for Force 9, and notes that the Met Office itself uses the descriptive term severe gale, as above.
Where the scale came from
Francis Beaufort, a Royal Navy officer and hydrographer, devised the scale in 1805. According to the Royal Meteorological Society it was officially adopted in the 1830s while he was Hydrographer of the Navy, and was first used on HMS Beagle under Robert FitzRoy.
How forecasts use it
- UK. The Shipping Forecast, prepared by the Met Office for the Maritime and Coastguard Agency, gives wind direction and Beaufort force for 31 sea areas four times a day, and gale warnings are issued for Force 8 or more.
- US. The National Weather Service issues warnings in knots rather than forces: a Gale Warning covers sustained winds or frequent gusts of 34 to 47 kt, a Storm Warning 48 to 63 kt and a Hurricane Force Wind Warning 64 kt or more, outside tropical cyclones. Small Craft Advisory thresholds vary by region.
The Met Office notes the internet is not part of the official Maritime Safety Information system. Read more in wind and weather.