NPS 1/4 Pipe

Outside diameter 0.540″

OD in mm 13.716 mm

NPT thread 1/4″ NPT (18 TPI)

Every NPS 1/4 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
40 = STD	0.088″	2.235	0.364″	9.246
80 = XS	0.119″	3.023	0.302″	7.671

1/4″ NPT thread

Designation	1/4″ NPT
Threads per inch	18
Recommended tap drill	7/16″ (0.4375″ / 11.112 mm)

NPT is a tapered thread (3/4″ per foot on diameter) — it seals on the thread flanks with sealant. The size refers to the pipe's nominal size, not any measured thread diameter.

Data provenance. ASME B36.10M, Welded and Seamless Wrought Steel Pipe (Table 2.1) (ASME). ASME B1.20.1, Pipe Threads, General Purpose (Inch) (ASME). Standard NPT tap drill chart (Industry reference). ID is derived: OD − 2 × wall. Dimensions are nominal; actual walls carry the standard's mill tolerance. See all data sources.