Sources and scope for this page: ISO 261:1998, covering the definition of thread pitch. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Metric threads specify pitch directly in millimetres; inch threads specify threads per inch. Converting one to the other is a reciprocal, not a scale factor: a 1.5 mm pitch is 16.9 TPI, and a 20 TPI thread has a 1.27 mm pitch.
Coarser pitch means fewer, deeper threads: faster to assemble, more tolerant of damage and plating, but a smaller minor diameter and therefore a weaker bolt. Fine pitch is the reverse.
Common misuse
Reading a metric pitch as a length and an inch pitch as a count without converting, which makes a 1.5 mm pitch look "coarser" than 20 TPI when it is in fact finer.