Threaded inserts are available with a range of thread sizes for compatibility with differently sized screws and bolts. Whether inserted into wood or metal, a threaded insert needs to match the diameter and threading of the screw or bolt that it will host.
The most used international screw thread standard is the ISO metric screw thread, as defined by international standard 68-1.
ISO metric screws are designated with the letter M, for millimetres, followed by the major diameter of the thread. This is the longest gap between the uppermost part of each thread (the crest). So, for example, M8 indicates a major diameter of eight millimetres.
Variations in pitch can also be indicated with a multiplication sign. For example, M8x1 would indicate a major diameter of eight millimetres with a change in pitch of one millimetre per rotation.