Wood screw size chart
Wood screws are sized by a gauge number: the bigger the number, the thicker the screw. Each step adds 0.013 inch to the diameter, starting from 0.060 inch at #0, so a #8 is 0.164 inch and a #10 is 0.190 inch. The length is given separately, in inches.
From the standard, ANSI B18.6.1, checked against four copies
| Gauge | Diameter (in) | Diameter (mm) | Threads per inch | Flat head, max (in) |
|---|---|---|---|---|
| #0 | 0.06 | 1.52 | 32 | 0.119 |
| #1 | 0.073 | 1.85 | 28 | 0.146 |
| #2 | 0.086 | 2.18 | 26 | 0.172 |
| #3 | 0.099 | 2.51 | 24 | 0.199 |
| #4 | 0.112 | 2.84 | 22 | 0.225 |
| #5 | 0.125 | 3.18 | 20 | 0.252 |
| #6 | 0.138 | 3.51 | 18 | 0.279 |
| #7 | 0.151 | 3.84 | 16 | 0.305 |
| #8 | 0.164 | 4.17 | 15 | 0.332 |
| #9 | 0.177 | 4.5 | 14 | 0.358 |
| #10 | 0.19 | 4.83 | 13 | 0.385 |
| #12 | 0.216 | 5.49 | 11 | 0.438 |
| #14 | 0.242 | 6.15 | 10 | 0.507 |
| #16 | 0.268 | 6.81 | 9 | 0.544 |
| #18 | 0.294 | 7.47 | 8 | 0.635 |
| #20 | 0.32 | 8.13 | 8 | 0.65 |
| #24 | 0.372 | 9.45 | 7 | 0.762 |
Reading the chart
The diameter is the basic size of the threads and the smooth shank under the head; a real screw may be a few thousandths under or over. A screw is named gauge first, then length: an 8 × 1-1/4 is a #8 screw 1-1/4 inches long. The head size is the largest a flat head may be with a sharp edge, the size its countersink has to reach. Flat and oval heads are cut at 82°.
The numbers are from the ANSI/ASME B18.6.1 wood screw standard. The standard is sold, so we checked the sizes against copies that agree: the USDA Wood Handbook’s table of shank diameters, Engineers Edge, Engineering ToolBox and Aspen Fasteners.
For the hole each screw needs, see the pilot hole chart or the pilot hole calculator.
Sources
- USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-190), ch. 8 Fastenings: lead holes and Table 8–9 screw shank diameters (as republished by CED Engineering, course S03-019), read 2026-11-07
- Engineers Edge, ANSI flat, pan and oval head wood screws per ANSI B18.6.1, size table; pilot hole drill sizes for wood screws, read 2026-11-07
- Engineering ToolBox, US wood screws: pilot holes, shank clearance holes and head diameters, read 2026-11-07
- Aspen Fasteners, Wood screws spec sheet: standard wood screw dimensions; 82° head angle, read 2026-11-07
- AmesWeb, Flat head wood screw sizes (ANSI B18.6.1-1981): conical head angle of approximately 82°, read 2026-11-07
- NIST, Special Publication 811, Appendix B (1 in = 25.4 mm)