Pilot Hole & Clearance Hole Size Calculator

How it works

Wood: pilot ≈ root diameter (softwood ≈ 0.7 × shank, hardwood ≈ 0.85 × shank); clearance = shank diameter. Machine screws: tapping drill ≈ major diameter − pitch (75% thread)

A pilot hole removes the wood the screw’s core would otherwise have to displace, leaving only the threads to bite, so the screw drives without splitting the board or snapping. The right size is roughly the diameter of the screw’s solid core: too small and hardwood splits or the screw shears, too large and the threads have nothing to grip. Softwood compresses, so it tolerates a smaller hole; dense hardwoods and ends of boards want a larger one. The top piece gets a clearance hole the full shank diameter so the screw pulls the pieces together rather than jacking them apart. For threaded holes in metal the equivalent is the tapping drill, which for standard 60° threads is simply the major diameter minus the pitch.

Worked example

A #8 screw (4.2 mm shank) into oak: 3.5 mm (9/64″) pilot in the hardwood, 4.2 mm (11/64″) clearance in the top board, and an 8–9 mm countersink. An M6 machine screw is tapped with a 5.0 mm drill.

Frequently asked questions