The European and international designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Nennweite), in which sizes are measured in millimetres, see ISO 6708.
Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others.
By 1949 thinner schedules 5S and 10S, which were based on the pressure requirements modified to the nearest BWG number, had been created, and other "S" sizes followed later.
Due to their thin walls, the smaller "S" sizes can not be threaded together according to ASME code,[4] but must be fusion welded, brazed, roll grooved, or joined with press fittings.
Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.
[a] Some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS), although these actually belong to an older system called iron pipe size (IPS).
Stainless steel pipe is most often available in standard weight sizes (noted by the S designation; for example, NPS Sch 10S).
DN does not exactly correspond to a size in millimeters, because ISO 6708 defines it as being a dimensionless specification only indirectly related to a diameter.