Windsock Sizing and Specification Guide

For licensed UK aerodromes, ICAO Annex 14 sets the minimum windsock size at 3.6 metres (12ft) in length with a mouth diameter of at least 0.9 metres (36 inches). For industrial COMAH sites, the right size depends on the primary viewing distance ; how far away is the person who needs to read the sock? For agricultural and general use, 3ft to 4ft is usually sufficient. Getting the size right is as important as getting the specification right: an undersized sock in the right specification is still the wrong product for the site.

This guide covers every standard size with mouth and tail dimensions, recommended applications and the regulatory specifications that apply to each.

Standard sizes and dimensions

The ICAO Annex 14 aviation specification

ICAO Annex 14, referenced by CAA CAP 168 for UK licensed aerodromes, sets minimum dimensions for wind direction indicators:

Minimum cone length: 3.6 metres (12ft). Minimum mouth diameter: 0.9 metres (36 inches). Colour: international orange. Performance: extending at 3 knots, fully extended at 15 knots.

The 12ft size with a 36-inch mouth is the universal aerodrome standard. On licensed UK aerodromes, this is a CAP 168 licensing requirement. On unlicensed private airstrips and gliding sites, 12ft remains the recognised best-practice benchmark.

For offshore helidecks under CAA CAP 437, size is assessed per installation rather than against a universal minimum, but ICAO Annex 14 specifications provide the reference.

Choosing size for industrial sites

On industrial sites, the driving factor for size selection is viewing distance, the maximum distance from which the sock needs to be readable.

Under 100 metres primary viewing distance: 4ft to 6ft is appropriate for most applications. A 4ft sock on a 3 metre mast is clearly readable at 80 to 100 metres from a muster point.

100 to 200 metres primary viewing distance: 6ft to 8ft gives the sock greater visual mass at distance and makes the angle of extension easier to judge accurately. At 150 metres a 4ft sock is still readable for direction but the speed indication from the angle is harder to assess precisely.

Over 200 metres primary viewing distance: 8ft to 12ft. For large industrial sites where the sock must be readable from the far end of a long evacuation route or from a crane cab at the far end of a large port, a larger sock ensures the reading is unambiguous at distance.

Multiple smaller socks versus one large sock: On very large sites, two or three 6ft socks positioned to cover different areas of the site often provides better practical coverage than a single 12ft sock in one position. Coverage of all muster points matters more than maximising the size of any single indicator.

Choosing size by sector

Licensed aerodrome: 12ft only. Non-negotiable for CAP 168 compliance.

Unlicensed airstrip or gliding site: 12ft best practice, 8ft minimum.

Onshore helipad: 4ft to 8ft depending on approach path geometry and aircraft types served.

Offshore helideck: Per CAP 437 assessment, typically 6ft to 12ft. Contact our team for offshore specification.

Oil and gas industrial site (onshore): 4ft to 8ft depending on site scale and viewing distances. COMAH upper-tier sites with large footprints typically need 6ft or 8ft for adequate coverage.

Chemical processing, EfW, waste: 4ft to 6ft for smaller sites; 6ft to 8ft for larger facilities with multiple process zones.

Water treatment works: 4ft to 6ft standard. The primary viewing requirement is from the chlorine storage area and the primary muster point.

Ports and marine construction: 6ft to 8ft for readability from crane cabs and vessel positions at distance.

Agriculture and spraying: 3ft to 4ft lightweight specification. Lighter fabric for better sensitivity at low wind speeds critical to spraying operations.

Events and temporary installations: 3ft to 4ft on a portable pole or ground spike.

Hardware compatibility, swivel arm matching

The mouth diameter determines which swivel arm the sock fits. A 15-inch mouth sock (3ft to 5ft sizes) fits a shorter swivel arm. A 24-inch mouth sock (6ft to 9ft) needs a longer arm. A 36-inch mouth (12ft) needs the full-length aviation arm.

When ordering a replacement sock, specify the size (length and mouth diameter) rather than just the length, and confirm hardware compatibility with the existing installation. A 6ft sock with a 24-inch mouth will not fit correctly on a swivel arm sized for a 15-inch mouth sock ; the mouth ring will not seat and the sock will not fly correctly.

Contact our team if you are unsure of your current installation’s swivel arm specification.

Frequently asked questions

What is the minimum size windsock for an aerodrome? ICAO Annex 14, referenced by CAA CAP 168, requires a minimum length of 3.6 metres (12ft) and a minimum mouth diameter of 0.9 metres (36 inches) for licensed aerodromes. This is the standard 12ft aerodrome specification used worldwide.

What size windsock do I need for a COMAH industrial site? Determined by the primary viewing distance from muster points and key operational positions. As a starting point: 4ft to 6ft for most smaller COMAH sites, 6ft to 8ft for larger sites where the primary viewing distance exceeds 100 to 150 metres. The sock must be clearly readable from every position where an emergency response decision will be made.

Does a larger windsock respond to wind differently? Slightly. Heavier fabric socks need marginally more wind energy to lift than lighter socks of the same specification but smaller size. For most applications this difference is negligible. For agricultural applications where sensitivity to very light breezes is critical, the smallest appropriate size in a lightweight fabric is the right choice.

Can I use a 6ft windsock instead of a 12ft on a licensed aerodrome? No. CAP 168 references ICAO Annex 14 which sets the 12ft minimum as a licensing requirement. A 6ft sock does not meet the specification and will be found non-compliant at aerodrome licensing inspection.

What is the difference between the 3ft to 5ft and the 6ft to 9ft mouth diameters? The 3ft to 5ft sizes use a 15-inch mouth diameter; the 6ft to 9ft sizes use a 24-inch mouth diameter. This reflects the need for a larger opening to inflate and fly a longer, heavier cone. The mouth diameter also determines swivel arm compatibility ; the two ranges require different swivel arm sizes.

How do I know which size sock my existing installation takes? Measure the mouth ring diameter. If it is approximately 15 inches (380mm) it is a 3ft to 5ft specification. If it is approximately 24 inches (610mm) it is a 6ft to 9ft. If it is approximately 36 inches (900mm) it is a 12ft aviation specification. If you cannot access the mast to measure, contact our team with the pole height and site type and we can advise on the most likely specification.