How to measure fasteners(bolts, screws)?
Measuring Fastener Diameter
For most types of fasteners, the diameter is measured on the outside of the threads.
Note: US diameters under 1/4” are given as numbers (e.g. #12) instead of inches, in order of increasing size. If you need to find the actual diameter, please refer to Table 2 in the Metric and Imperial Threads article.
Measuring Fastener Length
Fastener length is usually measured from where the material is assumed to be to the end of the fastener.
Thus, countersunk fasteners are measured overall and non-countersunk fasteners are measured from under the head.
Thread Count and Thread Pitch
Machine threaded fasteners specify a thread density in Threads Per Inch (US) or as a Thread Pitch in mm (Metric).
For a given diameter, a fastener may be available in coarse (standard), fine and sometimes super fine thread.