ASME
ASME B36.10M
Welded and Seamless Wrought Steel Pipe
The dimensional standard for carbon and alloy steel pipe: outside diameter and wall thickness against nominal size and schedule. It is where the fact that NPS 2 pipe is not two inches of anything comes from.
This is an explanation, not the standard. AlpineTutor is not affiliated with ASME and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.