Screw Size Converter
Convert screw sizes between metric (M2-M20), imperial gauge, and thread pitch.
Includes outer diameter and thread pitch reference for wood screws and bolts.
Select a metric or imperial screw size to see its equivalents, or enter a custom diameter.
Screw sizes differ between metric and imperial systems.
Metric screws are labeled by diameter in mm (e.g., M6 = 6 mm). The standard coarse pitch is fixed per size.
Imperial screws use gauge numbers (#0–#14) or fractional inches. Thread count is in TPI (threads per inch).
Common metric to imperial equivalents:
| Metric | Diameter | Nearest Imperial | Diameter |
|---|---|---|---|
| M2 | 2.0 mm | #2 | 2.18 mm |
| M3 | 3.0 mm | #4 | 2.85 mm |
| M4 | 4.0 mm | #8 | 4.17 mm |
| M5 | 5.0 mm | #10 | 4.83 mm |
| M6 | 6.0 mm | 1/4" | 6.35 mm |
| M8 | 8.0 mm | 5/16" | 7.94 mm |
| M10 | 10.0 mm | 3/8" | 9.53 mm |
| M12 | 12.0 mm | 1/2" | 12.70 mm |
| M16 | 16.0 mm | 5/8" | 15.88 mm |
| M20 | 20.0 mm | 3/4" | 19.05 mm |
Thread pitch conversion:
- Pitch (mm) = 25.4 / TPI
- TPI = 25.4 / Pitch (mm)
The imperial gauge numbers are the confusing part, because they’re an arbitrary scale rather than a measurement. A higher gauge number means a thicker screw, the opposite of wire gauge, and the numbers don’t map to any round figure: a #8 screw is 0.164 inches across, which is nothing you’d guess. Metric is refreshingly literal by comparison, where M6 simply means a 6 mm shank, which is why the rest of the world finds the gauge system baffling.
The equivalents in the table are nearest matches, not drop-in swaps. An M6 and a 1/4-inch screw are close in diameter, but their threads differ in pitch and angle, so they won’t interchange in a tapped hole or a threaded insert. It’s also worth separating two things the same number doesn’t tell you: diameter and length are specified independently, and a wood screw, a machine screw, and a sheet-metal screw of the same gauge have completely different thread shapes for completely different jobs. Match the diameter from this chart, then choose the thread type and length for the material you’re driving into.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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