How do I choose the right screw length?

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.

Do I always need a pilot hole?

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.

What is a clearance hole?

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.

What screw gauge should I use for 18mm board?

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.

WoodCalcs

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.

Top Piece Thickness

The piece the screw passes through

Bottom Piece Thickness

The piece the screw grips into

Wood Type

Number of Screws

Screws per Box (optional)

Price per Box (optional)

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.

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.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.