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Wood Screw Size Chart
Screw gauge numbers converted to shank diameter. Gauge describes the shaft diameter, not the length.
| Gauge | Diameter (in) | Diameter (mm) |
|---|
Standard: ANSI/ASME B18.6.1 · Data checked: 2026-10-14
Frequently asked questions
What does a #8 screw measure?
The number is a gauge, not a measurement. A #8 wood screw has a shank diameter of about 0.164 in, and the number does not scale linearly — going from #6 to #8 adds roughly 0.026 in. Always check the table rather than assuming a linear step.
What size pilot hole should I drill?
It depends on the screw size and on the wood, and the two matter equally. Softwood needs a smaller pilot than hardwood for the same screw, or the screw will not bite. A pilot hole that is too small splits the wood; too large and the screw has nothing to grip.
Why does a wood screw have a tapered shank?
The taper and the coarse thread are designed to displace wood fibres and pull the screw in, which is why wood screws have a sharp point rather than a machine screw's blunt end. Machine screws have a constant diameter because they thread into metal or a nut.
Are wood screws and drywall screws interchangeable?
No. Drywall screws are hard and brittle with a fine thread and a bugle head — they resist pull-through on plasterboard but snap if bent and hold poorly in structural timber. Use the screw designed for the job.