Drill Bit Size Calculator — Screw Pilot Holes

Pilot holes prevent wood from splitting, especially near edges and in hardwood. They also make driving screws easier and reduce the risk of the screw snapping. In softwood, a pilot hole is recommended; in hardwood, it is essential.

The clearance hole is drilled in the top piece of wood (the piece being fastened). It allows the screw shank to pass through without gripping, so the screw pulls the top piece tight against the bottom piece.

For countersunk screws (the most common type), yes. The countersink allows the screw head to sit flush with or below the wood surface. Skip the countersink for pan-head and round-head screws, and for coach screws, which are tightened with a spanner under their hex or square head.

Coach screws are sized by their metric thread diameter, not by wood-screw gauge. As a rule, drill the pilot hole to about half the thread diameter in softwood and about three-quarters in hardwood: an M8 coach screw needs roughly a 4mm pilot in softwood and 6mm in hardwood. Drill a full-diameter clearance hole through the top piece so the unthreaded shank passes freely (8mm for an M8).