Wood screw size chart
A #8 wood screw is 0.1640 in across. Drill a 7/64 pilot in softwood or 1/8 in hardwood.
Pilot holes are guidance. Published charts disagree and the right size depends on the species and its hardness, so test on a scrap of your board.
| Size | Diameter (in) | Pilot, softwood | Pilot, hardwood | Clearance |
|---|---|---|---|---|
| #2 | 0.0860 | 1/16 | 1/16 | 3/32 |
| #3 | 0.0990 | 1/16 | 5/64 | 7/64 |
| #4 | 0.1120 | 1/16 | 5/64 | 1/8 |
| #5 | 0.1250 | 5/64 | 3/32 | 9/64 |
| #6 | 0.1380 | 5/64 | 3/32 | 9/64 |
| #7 | 0.1510 | 3/32 | 7/64 | 5/32 |
| #8 | 0.1640 | 7/64 | 1/8 | 11/64 |
| #9 | 0.1770 | 7/64 | 1/8 | 3/16 |
| #10 | 0.1900 | 1/8 | 9/64 | 3/16 |
| #12 | 0.2160 | 9/64 | 5/32 | 7/32 |
| #14 | 0.2420 | 5/32 | 11/64 | 1/4 |
Pick a material in the drill bit finder to see the nearest common bit.
Gauge numbers and diameter
Wood screw gauges use the same arithmetic as numbered machine screws: diameter in inches is 0.060 + 0.013 x gauge, so #8 is 0.164 in. The head type and length are separate from the gauge.
Pilot hole or clearance hole?
The pilot hole goes into the board the screw bites into. It should be about the diameter of the shank under the thread, so the thread has wood to cut and the wood does not split. In hardwood it is larger than in softwood: for a #8 screw the chart gives 7/64 in softwood and 1/8 in hardwood. The clearance hole goes into the top piece, where the screw should slide through freely so the joint pulls tight.
Testing the hole
Hardness varies between species and even between boards, which is why published charts disagree. Drill a pilot in an offcut first. If the screw is hard to drive or the wood splits, go up one bit size; if the screw spins without gripping, go down one.