Glossary & Reference
Pitch diameter is the one thread dimension that actually decides whether a bolt and nut will fit together properly — more than the major diameter or minor diameter ever will on their own.
Every screw thread has three key diameters. Major diameter is the largest, measured crest to crest on an external thread. Minor diameter is the smallest, measured at the root of the thread. Pitch diameter sits between the two: it's the diameter of an imaginary cylinder positioned so that it slices through the thread at the point where the width of the thread ridge equals the width of the thread groove — in other words, right where the thread crosses exactly half its own depth.
Pitch diameter isn't something you can see or touch directly — it's a calculated, imaginary reference — but it's what actually controls how a male thread and a female thread engage. Two threaded parts can have acceptable major and minor diameters and still bind, gall, or fit too loosely if their pitch diameters aren't matched within tolerance. That's why thread gauging in a shop (with thread micrometers, three-wire measurement, or go/no-go thread gauges) is built almost entirely around checking pitch diameter, not the outer or inner diameter.
Thread standards such as Unified (UN/UNC/UNF) and ISO metric threads define an allowance and tolerance band on pitch diameter for each thread size and class of fit. A tighter class (like UNC Class 3A/3B) allows less pitch-diameter variation than a looser class (like Class 1A/1B), which changes how snug or how free the mating parts feel once assembled. Getting the pitch diameter right is what lets an off-the-shelf bolt from one manufacturer thread smoothly into a nut from a completely different one.
The same core idea shows up in gearing under a related name: the pitch diameter of a gear is the diameter of the theoretical "pitch circle" — the imaginary circle where two mating gears effectively roll against each other without slipping. It's the reference diameter used to calculate center distance, gear ratio, and tooth engagement, playing a similar role to thread pitch diameter: it's the working reference point, not a physical edge you can measure with calipers alone.
When you're cutting or chasing threads on a lathe, or running a tap or thread mill, pitch diameter is the dimension your process actually has to hit. Cut it oversize on an external thread (or undersize on an internal one) and the part may look right but won't assemble; get it within tolerance and the thread will function correctly even if the crest and root aren't perfectly finished.