What a wood screw gauge number actually means in inches and millimetres — and the part that saves your workpiece: the right pilot hole and clearance drill for softwood and hardwood. Drill the pilot and the wood won't split; skip it and it will.
Pilot hole = the small hole the threads bite into. Clearance (shank) hole = a larger hole in the top board so the screw pulls the two pieces tight instead of jacking them apart. Hardwood needs a slightly bigger pilot than softwood; always test in scrap first.
| Gauge | Ø inch | ≈ Fraction | Ø mm | Pilot · softwood | Pilot · hardwood | Clearance |
|---|
Gauge diameter is the standard wood-screw formula: Ø = 0.060″ + 0.013″ × gauge. Pilot and clearance drills are common twist-bit sizes; exact numbers vary by wood species and screw brand.
Roughly 3/32″ in softwood and 7/64″ in hardwood. The clearance (shank) hole in the top board is about 11/64″. A #8 screw itself is 0.164″ (about 4.2 mm) in diameter.
It's the shaft diameter, not the length. Bigger number = thicker screw. Each step adds 0.013″: a #6 is 0.138″, a #8 is 0.164″, a #10 is 0.190″. Length is the separate number in "#8 × 1-1/2″".
In hardwood, near an edge, or with larger screws — yes, or the wood splits. In soft pine with small screws you can often drive straight, but a pilot always gives a cleaner, stronger joint and protects against splitting.
The pilot hole is small — the threads cut into it for grip, drilled into the bottom piece. The clearance hole is larger, drilled through the top piece so the shank slides through and the screw clamps the boards together. Use both for a tight two-board joint.