Metric vs Imperial Fastener Sizing: Complete Thread Pitch & Dimension Chart

Demystifying Fastener Measurement Notations

Cross-threading a metric bolt into an imperial threaded hole or vice versa will permanently destroy the threads. Understanding the difference between Metric (ISO) and Imperial (Unified National - UNC/UNF) designations is fundamental for any technician or engineer.

How Metric Screws Are Designated

A metric fastener labeled M8 x 1.25 x 40 indicates:

• M8: Nominal outer shank diameter of 8 millimeters.
• 1.25: Thread Pitch — the distance between adjacent thread crests is 1.25 mm.
• 40: Usable length from under the head to the tip (40 mm).

How Imperial Fasteners Are Designated

An imperial bolt labeled 1/4"-20 x 1-1/2" indicates:

• 1/4": Nominal shank diameter (0.25 inches).
• 20: Threads Per Inch (TPI) — 20 complete thread turns in one inch.
• 1-1/2": Length under head (1.5 inches).

Standard Metric Coarse Pitch Quick Reference

Metric Size	Standard Pitch (Coarse)	Fine Pitch Option	Required Tap Drill Size
M3	0.50 mm	0.35 mm	2.5 mm
M4	0.70 mm	0.50 mm	3.3 mm
M5	0.80 mm	0.50 mm	4.2 mm
M6	1.00 mm	0.75 mm	5.0 mm
M8	1.25 mm	1.00 mm	6.8 mm
M10	1.50 mm	1.25 mm / 1.00 mm	8.5 mm
M12	1.75 mm	1.50 mm / 1.25 mm	10.2 mm