Screw size chart

Gauge, length and pitch are different measurements

In a wood screw marked #8 × 1 1/4 in, #8 is the size designation and 1 1/4 in is the specified screw length. A machine screw marked #8-32 additionally identifies 32 threads per inch. The gauge alone cannot determine the pitch or an appropriate nut.

This chart intentionally lists nominal major diameter rather than guessing a complete screw specification. A #8 reference diameter is 0.164 in, or 4.1656 mm. A nearby metric nominal size is not automatically an interchangeable thread.

Measure over the threads, not the head

The major diameter is the outside diameter across the screw threads. Head diameter, unthreaded shank, thread root, and clearance-hole size are separate measurements. Use calipers across several thread crests when checking an unidentified screw, then verify its thread form, pitch and other dimensions.

Keep the fastener family visible

Wood screws, machine screws and proprietary structural screws have different jobs and geometries. The wood-screw table includes gauges not included in the smaller common machine-screw reference. Selecting an unsupported gauge in machine mode produces a message instead of inventing a fit.

Use the pilot-hole tool for published preparation guidance for conventional wood screws. Do not use the major diameter as a pilot hole by default: doing so can remove the wood that the threads need to grip.

Sources and reference scope

Reference data checked November 7, 2026. Diameters are nominal dimensions over the outside of the threads, not measured tolerances or head dimensions.

• Bolt Depot · wood screw diameters
• Bolt Depot · US machine screw diameters