UK Screw Sizes — Gauges, Lengths and Pilot Holes

Screw gauges are a legacy imperial system still widely used in the UK. The gauge number roughly corresponds to the screw shank diameter. Common conversions: No.4 = 2.9 mm, No.6 = 3.5 mm, No.8 = 4.0 mm, No.10 = 4.8 mm, No.12 = 5.5 mm, No.14 = 6.3 mm. Modern screw packaging often shows both gauge and metric diameter, e.g., "4.0 x 50 mm" (which is a No.8 x 2" screw).

For softwood: the pilot hole should be approximately 50-60% of the screw shank diameter. For hardwood: approximately 75-80% of the shank diameter. For masonry plugs: the hole must match the plug size exactly (typically 5 mm for No.6-8, 6 mm for No.8-10, 7 mm for No.10-12, 8 mm for No.12-14, 10 mm for No.14+). Always match the wall plug colour/size to the screw gauge.

The No.8 (4.0 mm) screw is the most versatile and commonly used gauge in UK construction and joinery. In 40 mm length it is ideal for fixing skirting boards, architraves and light trim. In 50-75 mm lengths it handles general carpentry, battening, and fixing to studwork. The No.10 (4.8 mm or 5.0 mm) is used for heavier applications like decking and structural fixings.

Chipboard screws (also called multi-purpose or yellow zinc screws) have a coarse single-start thread that runs the full length of the shank. They are designed for chipboard, MDF, softwood, and general construction use. Traditional wood screws have a parallel unthreaded shank section and a tapered threaded section. Chipboard screws have largely replaced wood screws for most UK trade applications.

Pozi (Pozidriv) is the most common drive in UK construction and is denoted PZ1, PZ2, or PZ3. Phillips (PH) is similar but has a different cross pattern and is more common in electronics and imports. Torx (star drive, T20/T25/T30) is increasingly popular for decking and structural screws due to better cam-out resistance. Slotted (flathead) is still found but rarely used in new construction.