Thread Count (TPI)
TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch (see below). In general smaller fasteners have finer threads, so the thread count is higher. For a table of standard US thread counts please see our US Threads Per Inch Table.
Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. In general smaller fasteners have finer thread so they have lower thread pitch. For a table of standard metric thread pitchs please see our Metric Thead Pitch Table.