Thread size is measured in the U.S. inch or metric system, and conforms to common screw and bolt sizes. Thread size is identified by the external diameter, followed by the number of threads. For example, a 1”-8 tap means that it has a 1” external diameter and 8 threads.
For standard threaded fasteners, sizes less than 1/4” are assigned values from #0 to #16. Sizes 1/4” and larger are expressed in fractions. For example, a #8-15 fastener has a diameter smaller than 1/4" and 15 threads per inch. A 5/8”-11 fastener has a 5/8” diameter and 11 threads per inch.
For metric fasteners, the diameter is followed by the thread pitch, in millimeters per thread. For example, a M3-05 indicates a 3 millimeter diameter with 0.5 millimeters per thread pitch.