Thread Size Converter

Thread pitch and TPI are two ways to describe thread spacing.

The conversion:

• TPI = 25.4 / pitch (mm)
• Pitch (mm) = 25.4 / TPI

Common metric threads:

• M3: 0.5 mm pitch (50.8 TPI)
• M4: 0.7 mm pitch (36.3 TPI)
• M5: 0.8 mm pitch (31.75 TPI)
• M6: 1.0 mm pitch (25.4 TPI)
• M8: 1.25 mm pitch (20.3 TPI)
• M10: 1.5 mm pitch (16.9 TPI)
• M12: 1.75 mm pitch (14.5 TPI)

Common imperial threads (UNC):

• 1/4"-20: 20 TPI (1.27 mm pitch)
• 5/16"-18: 18 TPI (1.41 mm pitch)
• 3/8"-16: 16 TPI (1.59 mm pitch)
• 1/2"-13: 13 TPI (1.95 mm pitch)
• 5/8"-11: 11 TPI (2.31 mm pitch)
• 3/4"-10: 10 TPI (2.54 mm pitch)

The metric-to-imperial pairings here are approximate, not exact: an M6 is close to a 1/4-inch screw but not identical, so the two won’t interchange in a precision fit. That near-but-not-equal gap is exactly why mixing metric and imperial fasteners causes grief. Start an imperial bolt into a metric nut and it may catch, then cross-thread and ruin both.

Diameter is only half the spec, though, because pitch matters just as much. Metric threads come in coarse and fine versions (an M8 might be 1.25 mm pitch standard or 1.0 mm fine), and imperial threads are rated by threads per inch, like UNC coarse versus UNF fine. Two bolts of the same diameter but different pitch won’t thread together at all. When you’re matching a fastener you don’t recognize, a cheap thread pitch gauge settles it in seconds, which beats stripping a thread by guessing.