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Wood Screw Gauge Chart
Short answer
A wood screw gauge is its shank diameter, not its length: gauge diameter in inches is 0.060 + 0.013 x gauge, so a #8 is 0.164 in or about 4.2 mm, which is sold as a 4.0 mm screw in metric countries. The chart below also gives the pilot hole for softwood and hardwood.
Wood screw gauges, metric equivalents and pilot holes
| Gauge | Shank ⌀ (in) | Shank ⌀ (mm) | Pilot, softwood (mm) | Pilot, hardwood (mm) | Sold in metric as |
|---|---|---|---|---|---|
| #2 | 0.086 | 2.2 | 1.6 | 2.0 | 2.5 mm |
| #4 | 0.112 | 2.8 | 2.0 | 2.4 | 3.0 mm |
| #6 | 0.138 | 3.5 | 2.4 | 2.8 | 3.5 mm |
| #8 | 0.164 | 4.2 | 2.8 | 3.2 | 4.0 mm |
| #10 | 0.190 | 4.8 | 3.2 | 3.6 | 5.0 mm |
| #12 | 0.216 | 5.5 | 3.6 | 4.0 | 5.5 mm |
| #14 | 0.242 | 6.1 | 4.0 | 4.8 | 6.0 mm |
Gauge is diameter, length is separate
A screw sold as "8 x 1 1/2" is a #8 gauge, 1.5 inches long. The gauge fixes the shank diameter and the length is quoted separately, which is why a #8 x 1 inch and a #8 x 3 inch drive with the same bit and the same pilot hole.
The gauge formula is the same one the numbered machine screw sizes use: 0.060 in plus 0.013 in per gauge number. A #6 is 0.138 in, a #10 is 0.190 in.
Pilot holes: root diameter in softwood, one size up in hardwood
A pilot hole should clear the screw root so the threads cut wood instead of compressing it. In softwood, drill the root diameter. In hardwood, go one drill size larger — the wood has nowhere to move, and an undersized hole either snaps the screw or splits the board.
Near an end grain edge, add a countersink and clamp the joint. Most splits happen within two screw diameters of the end of a board, whatever the pilot hole.
Frequently asked questions
What size is a #8 wood screw in mm?
A #8 is 0.164 inch across the shank, which is 4.2 mm. Metric countries stock a 4.0 mm screw in its place; it is very slightly thinner and works in the same pilot hole.
What pilot hole should I drill for a #8 screw?
2.8 mm in softwood and 3.2 mm in hardwood. In softwood a pilot hole is optional away from edges, but in hardwood it is not.
Is a higher gauge number a bigger screw?
Yes, for wood and machine screw gauges — a #10 is thicker than a #6. That is the opposite of wire and sheet metal gauges, where a higher number is thinner, which is where the confusion comes from.
Other tools
Would rather just take a photo?
These charts are the same tables the app matches against. Screw Identifier does the measuring for you: photograph the fastener next to a ruler, or measure it on the screen against a calibrated on-screen caliper.
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