Thread chart

Which Thread?

The biggest problem for many people is to identify sizes of nuts and bolts. Diameter and length are easily established, but to identify the thread itself, a threadpitch gauge is the surest way, used in conjuction with the thread chart.

A good steel rule can be used as illustrated below to measure thread pitch. Imperial threads are referred to by threads per inch, and metric by the actual measurement between each thread.

Thread chart

Diameter BA UNF BSF BSW UNC Cycle Metric Metric Fine
1/8"/BA 38.5 - - 40 - - M3 x 0.5 -
3/16"/2BA 31.4 32 32 24 24 32 M4 x 0.7 (Common examples)
1/4" - 28 26 20 20 M6 M5 x 0.8 (Common examples)
5/16" - 24 22 18 18 26 M6 x 1.0 -
3/8" - 24 20 16 16 26 M8 x 1.25 1.0
7/16" - 20 18 14 14 20 & 26 M10 x 1.5 1.25 & 1.0
1/2" - 20 16 12 13 20 & 26 M12 x 1.75 1.5 & 1.25
9/16" - 18 16 12 12 20 & 26 M14 x 2.0 1.5
5/8" - 18 14 11 11 20 & 26 M16 x 2.0 1.5

How to measure fasteners

All types except countersunk.

How to measure thread