Nominal system
DN
DN is a dimensionless nominal designation. It is not a direct millimeter measurement of OD or ID.
HowNote Sizes · Pipe / Tube
Nominal size is not actual diameter.
HowNote keeps DN, NPS and regional A aliases separate from actual OD, wall thickness and calculated ID. The first public dataset is a deliberately limited Schedule 40 subset.
Nominal system
NPS
NPS is a nominal pipe size designation. Converting the NPS number directly to millimeters does not produce the actual OD.
Regional alias
A designation
A labels such as 50A are shown for regional convenience and are kept separate from official ASME terminology.