How do UK screw gauge numbers relate to metric sizes?

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).

What size pilot hole do I need?

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.

What is the most common screw size for general woodwork?

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.

What is the difference between chipboard screws and wood screws?

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.

What screw drive types are common in the UK?

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.

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UK Screw Sizes — Gauges, Lengths and Pilot Holes

UK screw gauge reference with metric and imperial sizes, pilot hole diameters for wood, masonry and metal, and common applications.

Screw Gauge to Metric Conversion

Gauge	Metric OD (mm)	Head Size (mm)	Pilot Hole Softwood	Pilot Hole Hardwood	Wall Plug Size
No. 4	2.9	5.5	1.5 mm	2.0 mm	5 mm (yellow)
No. 6	3.5	7.0	2.0 mm	2.5 mm	5 mm (yellow)
No. 8	4.0	8.0	2.5 mm	3.0 mm	6 mm (red)
No. 10	4.8	9.5	3.0 mm	3.5 mm	7 mm (brown)
No. 12	5.5	11.0	3.5 mm	4.0 mm	7 mm (brown)
No. 14	6.3	12.5	4.0 mm	5.0 mm	8 mm (brown)

Common Screw Lengths

Metric (mm)	Imperial	Common Application
16	5/8"	Hinges, light hardware
25	1"	Door furniture, thin boards
30	1.25"	Plasterboard to timber
40	1.5"	Skirting, architrave, general trim
50	2"	General carpentry, battening
65	2.5"	Decking boards, heavy trim
75	3"	Structural, door frames, heavy fixing
100	4"	Heavy structural, joists, purlins
150	6"	Structural timber connectors, sleepers

Related Calculators

Use the Screw Pattern Calculator for fixing spacings, or the Plasterboard Calculator for drywall screw quantities. See also the Fixings Reference for pull-out loads.

How We Calculate This

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.