Metric vs UNC Thread Pitch Comparison

Overview

Fundamental differences between metric and imperial thread pitch standards. While some diameters are close, the distance between thread crests (pitch) is distinct in each system.

Comparison Table

Nominal Size	Metric Pitch	UNC Pitch (TPI)	UNC Pitch (mm)	Difference
3mm / #4	0.50 mm	40 TPI	0.64 mm	0.14 mm
6mm / 1/4”	1.00 mm	20 TPI	1.27 mm	0.27 mm
8mm / 5/16”	1.25 mm	18 TPI	1.41 mm	0.16 mm
10mm / 3/8”	1.50 mm	16 TPI	1.59 mm	0.09 mm
12mm / 1/2”	1.75 mm	13 TPI	1.95 mm	0.20 mm

FAQ

Can I use metric threads in imperial holes?

No. Even if the diameter is nearly identical (like M8 and 5/16”), the thread pitch is different. Attempting to force them together will result in cross-threading and total joint failure.

Source Standard

ISO 965 / ASME B1.1