Screw size converter
A #8 wood screw measures 0.164" (4.2 mm) across the threads, close to M4. Type a gauge, a diameter in inches or mm, or a metric M-size, and get the nearest gauge from #4 to #14.
Entering a size in
Enter a wood screw gauge from #4 to #14, e.g. 8 or #10.
Enter the diameter in inches, e.g. 0.164 or 3/16.
Enter the diameter in millimetres, e.g. 4.2.
Enter a metric size, e.g. M4 or M5.
Nearest wood screw gauge
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How screw gauges work
A wood screw’s gauge is a code for its diameter across the threads. A higher number means a thicker screw: a #6 is thinner than a #8, which is thinner than a #10. The sizes follow diameter (inches) = gauge × 0.013 + 0.060, so a #8 is 8 × 0.013 + 0.060 = 0.164".
The screw’s size, not the hole’s
These are the sizes of the screw itself. The pilot hole you drill for it is smaller, and the countersink for a flat head is wider. For those, use the pilot-hole finder or the countersink size chart.
Metric sizes are near matches
An M-size is the screw’s diameter in millimetres, so an M4 is 4 mm across. Gauge and metric screws line up by diameter, but the thread pitch and the exact size differ, so the metric size is only the closest equivalent. Every gauge is in one table on the wood screw size chart, which also prints as a PDF.
Common questions
What is the diameter of a #8 screw?
A #8 wood screw is 0.164" across the threads, which is 4.2 mm. The nearest fraction is 5/32", and its flat head is about 11/32" (8.7 mm) across.
What is the diameter of a #10 screw in inches?
0.190", or 4.8 mm. The nearest fraction is 3/16".
What gauge is an M5 screw?
An M5 screw is 5 mm across. The closest wood screw gauge is #10, at 4.8 mm (0.190"). A #12 is 5.5 mm, a little bigger.
What are 8 and 10 gauge screws in metric?
A #8 is 4.2 mm across, about M4. A #10 is 4.8 mm, about M5.
What is the outer diameter of a #4 screw?
A #4 wood screw measures 0.112" (2.8 mm) across the threads. In metric that falls between M2.5 and M3, and the nearest fraction is 7/64".
Where we get our information from
Shank diameters are the ASME B18.6.1 basic sizes from a supplier’s wood screw thread sheet, which follow the gauge formula. Flat-head diameters are the “Actual Head Size” figures in W.L. Fuller’s countersink and taper-drill catalog. Millimetres, nearest fractions and nearest metric sizes are worked out from those, the same numbers as the wood screw size chart. Checked against published data from CDE Fasteners and W.L. Fuller.
See all 2 sources
- CDE Fasteners (supplier), wood screw thread and body diameters, per ASME B18.6.1
- W.L. Fuller, Countersinks, Counterbores, Tapered Drills, Plug Cutters catalog
These are standard nominal screw sizes. A real screw can measure a little off them, depending on the maker and the head style, so for a critical fit, measure your own screw.