General Questions and Answers

From what diameter a pipe in inches is equal to its actual outside diameter in millimeters?

Up to and including NPS 12 (12inch), the outside diameter does not correspond to the true diameter in millimeters.

Actual outside diameters
NPS 1 actual O.D. = 1.5/16" (33.4mm)
NPS 2 actual O.D. = 2.3/8" (60.3mm)
NPS 3 actual O.D. = 3.1/2" (88.9mm)
NPS 4 actual O.D. = 4.1/2" (114.3mm)
NPS 12 actual O.D. = 12.3/4" (323.9mm)
NPS 14 actual O.D. = 14" (355.6mm)