Screw Size Converter Calculator

Work out screw size converter instantly with clear inputs, formula shown and shareable results.

About Screw Size Converter Calculator

Imperial screw gauge relates to diameter by the linear rule 0.060 + 0.013 x gauge inches, so a number 8 screw is 0.164 inches or 4.17 mm - close to an M4. Lengths are quoted separately in inches, so a 8 x 1.5 inch screw is 4.17 mm by 38.1 mm in metric terms.

Formulas

Gauge to diameter

inches = 0.060 + 0.013 x gauge

Metric

mm = inches x 25.4

Worked example

With the default values below, Screw Size Converter Calculator returns Shank Diameter of 4.166. Change any input above and every figure here is recalculated in your browser.

Inputs used

Screw gauge numberminimum 0: 8
Screw lengthminimum 0 in: 1.5 in

Results

Shank Diameter: 4.166 mm
Same Diameter In Inches: 0.1640
Screw Length: 38.10 mm
Nearest Metric Thread: about M4

Step by step

1Values used
2Gauge to diameter
3Metric
4Shank Diameter= 4.166 mm
5Same Diameter In Inches= 0.1640
6Screw Length= 38.10 mm
7Nearest Metric Thread= about M4

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Is the gauge rule exact?

It is the standard approximation for wood screws. Machine screw standards specify exact diameters per size, which differ slightly.

Can I substitute M4 for a number 8?

For clearance holes usually yes, but threads are not interchangeable - pitch and thread form differ between the systems.