Pitch diameter is perhaps the most important element with regards to whether two parts will screw together. Pitch diameter is the distance from the pitchline to the corresponding pitchline located 180 degrees around.
All threads have a pitch diameter whether they are a straight or tapered thread. However, some threads’ pitch diameters are actually measured via their crest diameter (diameter from crest to crest) instead.
For ANSI, Metric, Premium or other straight threads:
Straight Thread Pitch Diameter
For API, Premium, or other tapered threads:
Tapered Thread Pitch Diameter
For API Tubing and Casing and select tapered threads:
Crest Diameter and Ovality