How this calculator works
A windsock fills and extends as the wind increases, and the number of segments held out gives a quick visual estimate of wind speed. The usual teaching rule treats each of five segments as about 3 knots, so one segment is about 3 knots, three segments about 9 knots and a fully extended sock about 15 knots or more. The actual calibration depends on the sock size, fabric and mounting, so use the number on the installation if one is provided. The estimate is for ground-level wind at the sock; wind at a different height or in a different exposure can differ.
Formula
Speed (kn) = segments extended x knots per segment. Speed (km/h) = knots x 1.852. Speed (m/s) = km/h / 3.6. Speed (mph) = km/h / 1.609344.
Worked example
Three segments extended at 3 knots each give 9 knots, which is 16.7 km/h, 4.63 m/s or 10.36 mph. A fully extended five-segment sock gives 15 knots, or 27.8 km/h.
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Frequently asked questions
How accurate is a windsock estimate?
It is a rough visual guide, not an instrument. Fabric stiffness, wetness, mounting height and the angle of view all affect how many segments appear extended. Use an anemometer for a number you will rely on.
What does a fully extended sock mean?
It means the wind is at or above the sock's rated upper speed, often about 15 knots for a standard five-segment sock. A fully extended sock cannot tell you how much stronger the wind is.
Why are windsock speeds given in knots?
Aviation and marine reports use knots, and one knot is one nautical mile per hour. The page also shows km/h, m/s and mph for weather and everyday use.
Does wind direction affect the number of segments?
No. The sock points downwind, so its extension reflects speed, not direction. Use a runway or compass reference separately to read the wind direction.