Wood screw sizes
7 screw gauges with their real diameter and the two different holes each one needs.
Gauge is a straight line
Screw gauge looks arbitrary and is not. The shank diameter in inches is 0.060 + 0.013 × gauge, so a #8 is 0.16 inches — 4.2 mm. Every gauge on this page comes from that one expression, and it reproduces the published tables to within the rounding they apply.
Metric screws are sold by their actual diameter instead, which is why a 4 mm screw is 4 mm and a #8 is not 8 of anything.
A joint needs two different holes
This is the part that decides whether two boards pull together. The screw passes through the first board and bites into the second, so:
- The top board gets a clearance hole — slightly wider than the shank, so the thread slides through without gripping.
- The bottom board gets a pilot hole — about two thirds of the shank in softwood, three quarters in hardwood, so the thread has material to cut into.
Drill a pilot hole in both and the screw jacks the boards apart. The thread grips the top piece as it turns, so instead of drawing the joint closed it pushes it open, and the gap gets blamed on the clamps. It is the most common reason a screwed joint will not close.
The full chart
| Gauge | Shank | Inches | Pilot, softwood | Pilot, hardwood | Clearance |
|---|---|---|---|---|---|
| #2 | 2.2 mm | 0.09 | 1.5 mm | 1.6 mm | 2.4 mm |
| #4 | 2.8 mm | 0.11 | 2 mm | 2.2 mm | 3 mm |
| #6 | 3.5 mm | 0.14 | 2.4 mm | 2.8 mm | 3.6 mm |
| #8 | 4.2 mm | 0.16 | 2.8 mm | 3.2 mm | 4.5 mm |
| #10 | 4.8 mm | 0.19 | 3.2 mm | 3.6 mm | 5 mm |
| #12 | 5.5 mm | 0.22 | 3.6 mm | 4.2 mm | 5.5 mm |
| #14 | 6.1 mm | 0.24 | 4.2 mm | 4.5 mm | 6.5 mm |
Why hardwood needs a bigger pilot
Softwood fibres compress around a screw, so a pilot hole at two thirds of the shank leaves enough material for the thread to bite while letting the wood give. Hardwood does not give — it splits. So the pilot goes up to about three quarters of the shank, which sounds like it should weaken the joint and does not: the thread is still cutting into solid material, and a split board holds nothing at all.
Near an end or an edge, go a size larger again. That is where splitting starts, and where a pilot hole matters most.
Metric threads — the machine-screw equivalent, with tapping drills. Lumber sizes · Spanner sizes