NPS 12 Pipe

Outside diameter 12.750″

OD in mm 323.85 mm

Every NPS 12 pipe has this OD regardless of schedule — the schedule changes only the wall thickness and bore.

Schedules

Schedule	Wall	Wall mm	Inside diameter	ID mm
STD	0.375″	9.525	12.000″	304.8
40	0.406″	10.312	11.938″	303.225
XS	0.500″	12.7	11.750″	298.45
80	0.688″	17.475	11.374″	288.9

Note the classic trap at this size: STD (0.375″) and Sch 40 (0.406″) are different walls — “Schedule 40” and “Standard” stop being synonyms at NPS 12.

Data provenance. ASME B36.10M, Welded and Seamless Wrought Steel Pipe (Table 2.1) (ASME). ID is derived: OD − 2 × wall. Dimensions are nominal; actual walls carry the standard's mill tolerance. See all data sources.