What is the 'Rule of 13' for screws?

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Screw Size Converter

Convert screw gauge numbers to major diameter in inches and millimeters.

Screw Gauge

Wood screw and sheet metal screw sizes are specified by gauge number. Machine screws use a similar numbering system for smaller sizes.

Major Dia (in)0.164

Major Dia (mm)4.17

Clearance Hole (mm)4.50

For wood screws, the major diameter (D) in inches is equal to **(Gauge × 0.013) + 0.060**. This allows you to estimate sizes between common gauges.

Always pre-drill a pilot hole slightly smaller than the root diameter of the screw to prevent splitting wood, especially in hardwoods or near edges.

Convert between screw gauges, fractional inches, and metric (mm) dimensions for wood and machine screws.

Select a standard screw gauge number.

Check the equivalent decimal inches and millimeter diameter.

The diameter in inches is roughly (Gauge x 0.013) + 0.060.

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Last updated on March 1, 2026