Thread Identification

Thread Identification


Identification of the thread size of unknown bolts or screws is relatively easy provided that it is done in a systematic way and with the correct basic equipment. There are only 2 majors’ screw threads systems that are used for fasteners (bolt &nuts) in the world today and there is a good chance that the bolt that needs identification with falls into one of these 2 categories:

  1. The unified thread system  (+- 60% in use in North America)
  2. The ISO Metric Screw

Basic tools needed for bolt thread identification

Vernier caliper – for measuring major diameter of bolt


Thread pitch (screw) gage (both inch & metric)

thread pitch
Inch/ Metric Scale – for measuring bolt length

regle boulons plus

3 Basics steps to follow for bolt thread-identification

  1. Measure the outside thread diameter (* major diameter) (image)
  2. Use the screw pitch age to identify whether the thread fits into the threads category. (image)
  3. Look up a thread chart to identify the thread combination of diameter / TPI or Pitch. (image)

*note: the major diameter of a bolt will always measure slightly less than the actual nominal diameter. This is because all bolts have a manufacturing tolerance and for bolts (external thread), this tolerance is on the minus side.


Téléphone: 450-420-1839