Screw Pilot Hole Calculator — Correct Drill Bit Sizes

Pilot holes are strongly recommended for hardwoods to prevent splitting. In softwood, smaller screws (No. 4-6) can often be driven without a pilot hole, but a pilot hole gives better results and reduces the risk of splitting, especially near edges or ends.

A clearance hole is drilled in the top piece of wood and is the same diameter as the screw shank. This allows the screw threads to pass through freely so the screw head can pull the two pieces tightly together.

Hardwood is denser and more prone to splitting, so the pilot hole is larger (about 75% of shank diameter). Softwood is softer and more forgiving, so a smaller pilot hole (about 60%) provides enough grip while allowing the screw to drive in easily.

Make it slightly bigger. Near an edge or end, or in dense timber, there is little material to resist the screw, so a tight pilot wedges the fibres apart and splits the wood. Enlarging the pilot by roughly 5–10% reduces that wedging force. Use the near-edge option to add about 7%.

Yes. Twin-thread chipboard/MDF screws have a thinner core and aggressive threads, so they need a smaller pilot (about 55% of shank, or none in soft chipboard). Coach (lag) screws are sized by metric shank diameter (M6–M12) rather than gauge and take a denser pilot — about 70% of shank in softwood and 85% in hardwood.