TPI stands for Threads Per Inch. TPI 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. In general smaller fasteners have finer threads, so the thread count is higher.
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.
Thread count or pitch is normally measured using a Thread Gauge. You can use our printable PDF thread gauge or you can buy a thread gauge. If you will be measuring fasteners frequently, it is recommended that you purchase one of our inexpensive thread gauges, which are significantly easier to use.