Case 1
- Wind speed
- 30km/h
5Bft
Open with these valuesBeaufort Scale Calculator
5Bft
Result: 5 BftEnter a wind speed and read the Beaufort force. The scale runs from 0, dead calm, to 12, hurricane force above 32.7 m/s. Its thresholds are defined in metres per second, so the speed is divided by 3.6 first — which is why 50 km/h still counts as force 6, a whisker below the 13.9 m/s that starts force 7.
Worked examples
Case 2
- Wind speed
- 50km/h
6Bft
Open with these valuesCase 3
- Wind speed
- 120km/h
12Bft
Open with these valuesHow it's calculated
m/s = km/h ÷ 3.6, then the highest force whose lower bound is at or below it
- StepEnter the mean wind speed in km/h — switch the units above for mph.
- StepThe speed is divided by 3.6 to get metres per second.
- StepThe result is the highest force whose lower bound does not exceed it.
- ResultGusts are not covered; the scale uses the ten-minute mean.
Reference table
| Wind speed | Name and lower bound | Force |
|---|---|---|
| 0 | Calm · from 0 m/s | 0 |
| 2 | Light air · from 0.5 m/s | 1 |
| 6 | Light breeze · from 1.6 m/s | 2 |
| 13 | Gentle breeze · from 3.4 m/s | 3 |
| 20 | Moderate breeze · from 5.5 m/s | 4 |
| 29 | Fresh breeze · from 8 m/s | 5 |
| 39 | Strong breeze · from 10.8 m/s | 6 |
| 51 | Near gale · from 13.9 m/s | 7 |
| 62 | Gale · from 17.2 m/s | 8 |
| 75 | Strong gale · from 20.8 m/s | 9 |
| 89 | Storm · from 24.5 m/s | 10 |
| 103 | Violent storm · from 28.5 m/s | 11 |
| 118 | Hurricane force · from 32.7 m/s | 12 |
Questions
What is the Beaufort scale?
The Beaufort scale is a 13-step system, force 0 to 12, that classifies wind strength by its observable effects on land and sea. It was devised by Admiral Sir Francis Beaufort in 1805 and is the standard in marine and meteorological forecasts worldwide. Force 0 is dead calm; force 12 is hurricane force above 32.7 m/s.
How do I convert km/h to Beaufort?
Divide the speed in km/h by 3.6 to get metres per second, then take the highest Beaufort force whose lower bound does not exceed it. For instance 30 km/h is 8.33 m/s, which sits between the force 5 bound of 8 m/s and the force 6 bound of 10.8 m/s, so the answer is force 5.
Why does 50 km/h come out as force 6 and not 7?
Because the scale is defined in metres per second, and 50 km/h is 13.89 m/s — a hair under the 13.9 m/s where force 7 begins. Published km/h columns round that boundary down to 50 and therefore list 50 km/h as force 7. This calculator follows the metres-per-second definition, so the two disagree on exactly that value.
What does each Beaufort force feel like?
Force 0 to 1 is calm to barely perceptible; force 3 rustles leaves; force 6 makes an umbrella hard to control; force 8 makes walking against the wind hard; force 10 can strip roofs and uproot trees; force 12 causes catastrophic structural damage.
Does the Beaufort scale measure gusts?
No — it is based on the ten-minute mean wind speed measured 10 m above open ground. Short-lived gusts are not counted and can be significantly higher than the mean. Check gust data separately, especially for sailing or work outdoors.
What wind speed is Beaufort force 12?
Force 12, hurricane force, begins at 32.7 m/s, which is about 118 km/h. At that intensity the air is filled with foam and spray, visibility is severely reduced and structural damage is widespread. In practice force 12 is only reported in tropical cyclones and the most severe extratropical storms.
Sources and last check
Information, not professional advice.