Wood Screw Size Guide
Wood screws in the UK are commonly sold by gauge number (the traditional sizing) and length. The gauge determines the shank diameter, which in turn dictates the pilot hole size you need. Drilling the correct pilot hole prevents splitting, ensures a strong hold and makes driving the screw much easier.
Screw Gauge to Diameter
| Gauge | Shank Diameter (mm) | Head Diameter (mm) | Pilot Hole — Softwood (mm) | Pilot Hole — Hardwood (mm) | Clearance Hole (mm) |
|---|---|---|---|---|---|
| 4 | 2.9 | 5.5 | 1.5 | 2.0 | 3.0 |
| 5 | 3.2 | 6.0 | 1.5 | 2.5 | 3.5 |
| 6 | 3.5 | 6.5 | 2.0 | 2.5 | 4.0 |
| 7 | 3.8 | 7.0 | 2.0 | 3.0 | 4.0 |
| 8 | 4.2 | 8.0 | 2.5 | 3.0 | 4.5 |
| 9 | 4.5 | 8.5 | 2.5 | 3.5 | 5.0 |
| 10 | 4.8 | 9.0 | 3.0 | 3.5 | 5.0 |
| 12 | 5.5 | 10.0 | 3.5 | 4.0 | 6.0 |
| 14 | 6.3 | 11.5 | 4.0 | 4.5 | 6.5 |
The pilot hole is drilled in the piece that receives the screw thread. The clearance hole is drilled through the top piece so the screw pulls the two pieces tightly together. Always drill a pilot hole in hardwood to prevent splitting.
Common Screw Lengths
| Length (mm) | Length (inches) | Typical Applications |
|---|---|---|
| 12 | ½″ | Thin sheet materials, hinges |
| 16 | ⅝″ | Thin panels, light fixings |
| 20 | ¾″ | Cabinet hardware, thin boards |
| 25 | 1″ | General joinery, shelving |
| 30 | 1¼″ | Cabinetry, furniture assembly |
| 40 | 1½″ | General construction, framing |
| 50 | 2″ | Heavier construction, decking |
| 65 | 2½″ | Decking, structural connections |
| 75 | 3″ | Heavy framing, posts |
| 100 | 4″ | Structural timber connections |
As a rule of thumb, the screw should penetrate at least two-thirds of its length into the receiving piece. For example, when fixing a 20 mm board, use at least a 50 mm screw so 30 mm+ enters the material below.
Related Calculators
- Adhesives & Fasteners Calculators — screw quantity estimation, spacing guides and more
Pilot hole sizes are general recommendations and may need adjusting for very hard or very soft species. Modern construction screws (e.g. Spax, Reisser) often have self-drilling tips that reduce the need for pilot holes in softwood, but a pilot hole is always recommended in hardwood.