Sources and scope for this page: ASME B1.1-2019, covering the definition of pitch diameter. 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
The pitch diameter is the dimension that actually controls whether two threaded parts assemble correctly, and it is the one a thread micrometer or three-wire measurement checks. The major diameter is easier to measure but says much less — a thread can be within tolerance on major diameter and still not fit.
For a 60° thread, d₂ = d − 0.64952 × P, where d is the major diameter and P the pitch. This is why a finer pitch gives a larger pitch diameter and therefore a stronger bolt.
Common misuse
Believing the major diameter defines a thread. Two threads with identical major diameters can differ in pitch, pitch diameter and therefore in whether a nut will go on.