Pilot Hole Size Chart

The rule behind every pilot hole

A screw holds because its thread displaces and grips material. The pilot hole exists to remove just the material the solid core of the screw needs to occupy, and no more. So the target is the root diameter, the shaft measured at the bottom of the threads, not the major diameter measured across the thread crests.

From there, adjust for how the material behaves:

• Softwood: about 70 percent of the major diameter. The fibres compress and grip well.
• Hardwood: about 85 to 90 percent. Dense timber will not compress, so an undersized hole splits it or snaps the screw.
• Particle board and MDF: close to the root diameter, and always pilot near edges.
• Steel sheet: close to the root diameter, since the thread must form in metal that will not compress.

Wood screw pilot hole chart

Clearance hole is drilled through the top piece only, so the screw passes freely and the head pulls the joint closed. Sizes suit our wood screw range (Ø3 to 8 mm).

Chipboard and MDF pilot hole chart

Engineered panels split easily along their edges and ends because there is no continuous grain to resist the wedging action of the thread. Piloting is cheap insurance.

For our chipboard screws (Ø3 to 6 mm). A Type 17 chip-removal point reduces splitting further by clearing material ahead of the thread.

Self tapping screw pilot holes in steel sheet

In sheet metal the hole must be tight enough for the thread to form properly but loose enough that driving torque stays below the screw's torsional strength. Thicker gauge needs a slightly larger hole.

Guide figures for mild steel, matching our self tapping screw range. Stainless and higher-strength sheet usually need the hole at the upper end of the range. Always trial in your actual substrate and gauge.

Tap drill sizes for metric machine screws

Machine screws do not form their own thread. The receiving hole is drilled and then tapped, so the drill size is set by the pitch rather than by the material.

Clearance holes per ISO 273 medium fit. Suits our machine screw range, M2 to M16.

Common pilot hole mistakes

• Drilling the pilot through both pieces. The top piece needs a clearance hole. If the thread grips it too, the two parts cannot be pulled together and you get a gap in the joint.
• Using the same hole for hardwood and softwood. Oak and beech need a hole roughly 20 percent larger than pine for the same screw.
• Ignoring the countersink. A countersunk head that cannot seat flush will split the surface as it is forced down. Countersink separately or use a bugle head that self-countersinks.
• Piloting to the wrong depth. The hole should be at least as deep as the screw penetration, otherwise the point bottoms out and torque spikes.
• Not piloting near edges. Within about 50 mm of an edge or end, splitting risk rises sharply in both timber and engineered board.

Specify with confidence

These charts cover the sizes we manufacture. For the full dimensional picture see the screw size chart, and for head and recess selection see the head types and drive types guides.

Bulk buyer, OEM or importer? Send your specification and quantity for a quote, or ask about white-label supply under your own brand.