A windsock answers one question exactly and a second question only roughly. The exact answer is direction: the sock points downwind, so the wind is coming from the mast end, and that is the whole reason it exists at an airfield. The rough answer is speed, read from how far the sock hangs below horizontal, and this windsock wind speed calculator turns that angle into knots using the small number of calibration points that are actually published.
Arb Digital wrote it as an interpolation between three documented figures rather than as a physical model, and the distinction matters. A drag-against-weight model of a hanging cone produces a curve that does not match the published points, which tells you that the real behaviour depends on how the sock inflates, how heavy the fabric is and how the swivel behaves. Rather than invent a law and dress it up, the tool uses the published numbers and says openly where it has run out of them.
What This Calculator Does
It takes the angle you observe and interpolates between three calibration points: fully extended at 15 knots, five degrees below horizontal at 10 knots, and thirty degrees below horizontal at 6 knots. It reports the result in knots, kilometres per hour, miles per hour and metres per second, and gives the corresponding Beaufort force.
All three calibration points are editable, because the numbers apply to a conforming aviation wind cone and a great many socks are not one. It also refuses two cases outright. Below the lowest calibration point — a sock hanging more than thirty degrees down — there is no published relation, and the tool says so instead of extrapolating. At full extension it reports a floor rather than a figure, because a fully extended sock looks identical at 15 knots and at 40.
How to Use It
- Judge the angle of the whole sock, not the tip. A partly filled sock droops at the tail while its axis sits higher, and reading the tail overestimates the droop.
- Look at it from the side, at ninety degrees to the wind if you can. Viewed from downwind or upwind the foreshortening makes the angle almost impossible to judge.
- Leave the calibration points alone for a standard aviation wind cone and change them only if you know your sock is non-conforming.
- Count fully extended stripes as a cross-check if the sock is striped, remembering that this is a rule of thumb rather than part of any specification.
- Watch it for thirty seconds. A sock moves constantly. One glance gives you a gust or a lull, not the mean wind.
The Published Figures
Wind cones at aerodromes are built to a specification. The FAA advisory circular covering them, AC 150/5345-27F, Specification for Wind Cone Assemblies, requires that a conforming cone extend fully in a 15-knot wind and orient itself in a wind of at least 3 knots. Those two figures bracket the useful range: below 3 knots the sock cannot even tell you the direction reliably, and above 15 it has nothing left to say about speed.
Between those bounds, the commonly reproduced intermediate figures come from Transport Canada and are summarised in the general reference on the windsock: a 10-knot wind raises the sock to 5 degrees below horizontal and a 6-knot wind to 30 degrees below. The same reference notes the stripe convention, in which each fully extended stripe of a striped sock is read as roughly 3 knots.
The Beaufort force in the results uses the National Weather Service’s published Beaufort wind scale, on which force 3 runs from 7 to 10 knots and force 4 from 11 to 16.
Worked example on the defaults. An angle of 15 degrees below horizontal sits between the 5-degree point at 10 knots and the 30-degree point at 6 knots. The fraction of the way across is (15 − 5) ÷ (30 − 5) = 0.4, so the speed is 10 + 0.4 × (6 − 10) = 10 − 1.6 = 8.4 knots. Converted, that is 8.4 × 1.852 = 15.6 km/h, 8.4 × 1.15078 = 9.7 mph and 8.4 × 0.51444 = 4.3 m/s, and 8.4 knots falls in Beaufort force 3.
Why the Model Is Interpolation Rather Than Physics
The obvious way to model a windsock is as a balance between horizontal drag and vertical weight, giving an angle whose tangent goes as one over the square of the wind speed. Fit that model to the 6-knot, 30-degree point and it predicts about 12 degrees at 10 knots, where the published figure is 5. The single-parameter physical model is simply wrong, and the reason is that a windsock is not a rigid object hanging in a stream.
It is a fabric cone that inflates. As wind speed rises the sock fills, its shape changes, its effective drag area changes with it, and the fabric goes from limp to taut. That is a far messier problem than a pendulum in a breeze, and it depends on the specific sock’s fabric weight, its taper, the size of its throat and how freely its swivel turns. Publishing a clean-looking formula would give an answer that is precise and wrong. Interpolating between measured points gives an answer that is rough and honest, which is the right trade for a device whose whole purpose is a quick visual impression.
Where the Reading Breaks Down
Three situations make the estimate unreliable, and it is worth knowing all three.
Above full extension. A sock at 15 knots and a sock at 40 knots both fly straight out. There is no more information in the picture, and any tool claiming to distinguish them from angle alone is inventing it. All you can honestly say is that the wind is at or above the extension speed.
Below the lowest calibration point. Below about 6 knots the published relation stops, and the sock is in the region where it is beginning to lose its ability to orient at all. The FAA figure of 3 knots is a threshold for direction, not a calibration for speed. This tool refuses to produce a number there, and the honest reading of a sock hanging steeply is simply “light wind”.
Sock condition and siting. A wet sock is heavier and hangs lower for the same wind. A faded, frayed or waterlogged sock reads low, a stiff new one may read high, and a swivel that has seized reads nonsense in both direction and angle. Siting matters as much: a sock in the lee of a hangar, a treeline or a hill is reporting the local eddy, not the wind over the runway, which is why aerodromes site them in clear air and often have more than one.
What to Use Instead When It Matters
For anything with a consequence, the windsock is a cross-check rather than a source. Reported winds from an automated station, an ATIS broadcast or an anemometer give a measured figure with a known averaging period, and they distinguish the mean from the gust, which a sock cannot do at all — the difference between a steady 15 and a 10 gusting 25 is the difference between a routine landing and a difficult one, and both look similar on a sock across an airfield.
What the sock does supply, better than any instrument, is instant direction relative to the runway you are looking at. Once you have both direction and speed, the crosswind component calculator splits them into the headwind and crosswind components for any heading, which is the number that actually decides things.
Arb Digital builds free calculators that cite the specification, mark the guesswork and refuse the cases they cannot do properly. Browse the library, or tell us what your readers keep getting wrong.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Reading the tail instead of the axis. A partly filled sock droops at the tip while its main body sits higher, and following the tip exaggerates the angle and understates the wind.
- Judging the angle from upwind or downwind. Foreshortening makes a sock pointing at or away from you unreadable. Move so you are looking across the wind.
- Treating a fully extended sock as exactly 15 knots. It is a floor. The sock is saturated and cannot distinguish 15 from 40.
- Taking one glance. Wind is gusty and a sock is always moving. Thirty seconds of watching gives you a mean and a range; a snapshot gives you neither.
- Ignoring where the sock is standing. A sock sheltered by a building, a treeline or terrain reports the eddy around it rather than the wind you care about.
Related Free Tools From Arb Digital
Split a wind into its headwind and crosswind parts for a given runway with the crosswind component calculator, and move between knots, kilometres per hour, miles per hour and metres per second in the speed converter. For the effect of wind on people, the wind chill calculator applies the official formula; for its effect on structures, the wind load calculator works from pressure. The wind turbine calculator covers energy in a moving air stream, and the angle converter handles degrees, radians and gradients. Everything else is in the free online tools hub.
Frequently Asked Questions
By the angle it hangs below horizontal. A conforming aviation wind cone is fully extended at 15 knots, sits about 5 degrees below horizontal at 10 knots and about 30 degrees below at 6 knots. Between those points the estimate is interpolation, and outside them the published relation does not apply.
At least 15 knots for a conforming cone, and possibly far more. Full extension is a floor rather than a measurement: the sock has no further movement available, so it looks the same at 15 knots as at 40. Use a reported or measured wind if the difference matters.
By common convention each fully extended stripe is read as roughly three more knots of wind. It is a field rule of thumb rather than part of the wind cone specification, which is why this tool offers it as a cross-check rather than as the headline figure.
Because the published calibration points stop there. Below about 6 knots there is no documented angle-to-speed relation, and the FAA figure of 3 knots is the minimum wind that lets a sock indicate direction at all, not a calibration for speed. Extrapolating past the data would produce a number with nothing behind it.
For direction, yes, which is its actual job. For speed it is an indication rather than a measurement, and it is affected by fabric weight, whether the sock is wet, wear and how freely the swivel turns. It also cannot separate a steady wind from a gusty one, which is often the more important distinction.
Yes. Water adds weight without adding drag area, so a soaked sock hangs lower for the same wind and reads slow. Faded, frayed and waterlogged socks all read low, which is one reason wind cones are replaced on a schedule rather than when they fail.
Because that model does not fit the published points. Calibrated to the 6-knot figure it predicts roughly 12 degrees at 10 knots where the published value is 5. A windsock is an inflating fabric cone whose drag area changes with speed, not a rigid body in a stream, so interpolating measured points is more honest than a tidy formula that is wrong.
Only with the calibration points changed, and even then with caution. The 15-knot extension figure applies to a wind cone built to the aviation specification. A lighter decorative sock extends in far less wind and a heavy industrial one in more, so the same angle means different speeds on different socks.
This page produces a rough visual estimate, not a measurement. A windsock is a wind direction indicator; it is not an anemometer, it cannot distinguish mean wind from gusts, and its reading is affected by fabric condition, wetness, wear and where the sock is sited. Do not use an estimate from this page as the basis for any flight, lifting, spraying or other operational decision — use the reported or measured wind and follow the procedures of the relevant aviation or safety authority.
