Thread Pitch Reference Chart — UN & Metric

What it is

Every screw thread has a diameter and a pitch. Unified inch threads (ASME B1.1) state pitch as threads per inch (TPI) and come in coarse (UNC) and fine (UNF) series. ISO metric threads (ASME B1.13M, ISO 261) state pitch as the distance between threads in millimeters. A metric size with no pitch given is coarse.

How to identify

Measure the major diameter with calipers. Then check pitch with a thread pitch gauge: find the leaf that seats with no light between the teeth. Inch and metric gauges can almost fit each other on some sizes, so check along several threads.

Chart

Rows pair inch and metric sizes only to save space; the sizes on one row are not equivalents.

Key points

• #10-24 and #10-32 are both common; #10-32 is the fine thread.
• #0 has only a fine thread (80 TPI).
• Coarse threads assemble faster and resist cross-threading and damage. Fine threads allow finer adjustment and have a slightly larger stress area.
• Extra-fine (UNEF), 8-pitch (8UN) and other constant-pitch series exist beyond this chart.

Common mistakes

• Forcing an M8 × 1.25 bolt into a 5/16-18 hole, or an M6 × 1.0 into a 1/4-20 hole. They start, then bind and strip.
• Calling a metric fine thread by size only, which suppliers will read as coarse.
• Reading a worn pitch gauge on a damaged thread.

Governing standards: ASME B1.1 (Unified inch screw threads); ASME B1.13M (metric screw threads, M profile); ISO 261 (metric general-purpose threads, general plan).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.