General Questions & 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)

As you can note, the outside diameter from NPS 14 is equal to the true diameter in millimeters.

Pipe Plain Ends

Basically, this website does not use the term Inch for pipe, fittings and flanges, but NPS (Nominal Pipe Size).

Detailed Information

Pipe Beveld Ends
Nominal Pipe Size
Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures...

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