WoodCalcs.uk
Screw Size Calculator — Find the Right Wood Screw
Determine the correct screw gauge, length, and pilot hole size based on your wood thickness and species.
How We Calculate This
This calculator recommends screw size based on established woodworking guidelines for joint strength and split prevention.
Screw length selection
Ideal length = Top piece thickness + (Bottom piece x 0.67)
The result is rounded up to the nearest standard UK screw length, then capped if necessary so the tip stays at least 5mm clear of the far face (no break-through).
Pilot hole sizing
Pilot holes for softwood are smaller (60-70% of shank diameter) than for hardwood (75-85% of shank diameter). The calculator adjusts automatically based on your wood type.
Working figures for this are collected in our Screw Size Guide reference table.
Frequently Asked Questions
The screw should pass through the top piece completely and penetrate 60-75% into the bottom piece. This gives maximum holding power without risking the screw poking through. Never let the screw come within 5mm of the bottom face.
In hardwood, MDF, and plywood, always drill a pilot hole to prevent splitting. In softwood, pilot holes are recommended for screws near edges or ends, and for gauges 8 and above. Self-drilling screws can skip pilot holes in softwood.
A clearance hole is drilled through the top piece only. It is slightly larger than the screw shank so the screw passes through freely, pulling the top piece tight against the bottom piece. This is essential for a proper screw joint.
For 18mm sheet materials like MDF or plywood, gauge 6 (3.5mm) or gauge 8 (4mm) screws are most common. Use 30-40mm length screws depending on what you are screwing into. Always drill pilot holes in MDF to prevent splitting.
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Last updated: February 2026
All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.