Choosing Screws and Drilling Pilot Holes
Screw gauges, the length rule and pilot-hole sizes that stop wood from splitting.
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Screw gauge to diameter
Screw sizes are sold by gauge. Approximate diameters:
| Gauge | Diameter |
|---|---|
| 6 | 3.5 mm |
| 8 | 4.2 mm |
| 10 | 4.8 mm |
| 12 | 5.5 mm |
| 14 | 6.3 mm |
The length rule
A screw should penetrate the substrate about two thirds of its length. A simple rule:
Recommended length ≈ 2 × thickness of the top board
For an 18 mm top board, use ~35–40 mm screws.
Pilot hole sizes
The pilot hole lets the threads cut instead of the wood splitting:
Thread pilot ≈ 60% of diameter (softwood)
Thread pilot ≈ 85% of diameter (hardwood)
Shank pilot ≈ 95% of diameter
| Screw | Softwood pilot | Hardwood pilot |
|---|---|---|
| 3.5 mm (g6) | 2.1 mm | 3.0 mm |
| 4.2 mm (g8) | 2.5 mm | 3.6 mm |
| 4.8 mm (g10) | 2.9 mm | 4.1 mm |
Practical tips
- Always pilot near edges and ends, and in hardwood.
- Countersink so heads sit flush.
- Wax or soap the screw to ease driving in dense woods.
Get exact sizes for your screw with the screw & pilot hole calculator.
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