Pipe Size Converter (NPS to MM)

Find the real outside diameter of a nominal pipe size and its metric DN equivalent — for steel and PVC pipe.

How to use the Pipe Size Converter (NPS to MM)

Choose the nominal pipe size.
Read the outside diameter in inches and mm.
Note the metric DN equivalent.

How it works

Nominal pipe size (NPS) is a label, not a measurement: a 1″ pipe has an outside diameter of 1.315″. For each NPS the outside diameter is fixed so fittings interchange; the wall thickness, and therefore the inside diameter, depends on the schedule (40, 80, 160). The metric equivalent is DN (diamètre nominal) in millimeters. Copper tube uses a different system.

NPS outside diameters

NPS	OD (in)	OD (mm)	DN
½	0.840	21.3	15
¾	1.050	26.7	20
1	1.315	33.4	25
1¼	1.660	42.2	32
1½	1.900	48.3	40
2	2.375	60.3	50
3	3.500	88.9	80
4	4.500	114.3	100

Frequently asked questions

What is the outside diameter of 1/2″ pipe?

0.840 in (21.3 mm).

What is DN 50 in inches?

NPS 2 (2.375″ OD).

Why is a 1″ pipe not 1″?

Nominal sizes date from early iron pipe and refer roughly to inside diameter, not outside.

Is PVC pipe the same size as steel pipe?

Schedule 40/80 PVC uses the same outside diameters as steel pipe.