What is the difference between threads per inch and thread pitch? Threads per inch, or TPI, and thread pitch are different but related methods of measuring the position of the threads on a screw, bolt, or fastener. The thread is the helical protrusion that is found along the length of these parts. TPI is a numerical representation of the number of threads in every inch of length along a screw. The thread pitch is a measurement of the distance between two thread peaks. Thread pitch is used when measuring or referring to metric parts. Both the TPI and thread pitch of a screw, bolt, or fastener can be converted between the alternative value using conversion tables, calculators, or a mathematical formula. Knowing the TPI or thread pitch of a particular part is important information for knowing whether the part will fit into the space it is supposed to go.