DN vs NPS: Pipe Size Designations and the Outside-Diameter Table

DN50, NPS 2 and 60.3 mm all name the same pipe. How the metric and inch nominal systems correspond, and the confusions that cost money.

Two names, one table

NPS (Nominal Pipe Size) is the North American designation, in inches. DN (diamètre nominal) is the metric designation used with EN and ISO standards. Neither is a measurement: both are labels that point to a row in a standard outside-diameter table (ASME B36.10M in the NPS world, EN 10220 Series 1 in the DN world — and the two tables agree).

The correspondence

DN	NPS	Outside diameter (mm)
DN 15	1/2	21.3
DN 20	3/4	26.9
DN 25	1	33.7
DN 40	1 1/2	48.3
DN 50	2	60.3
DN 80	3	88.9
DN 100	4	114.3
DN 150	6	168.3
DN 200	8	219.1
DN 250	10	273.0
DN 300	12	323.9
DN 350	14	355.6

The classic example

DN50 = NPS 2 = 60.3 mm OD. The "50" suggests a 50 mm bore and the "2 inch" suggests 50.8 mm — the actual pipe is neither. Its bore depends on the wall: in Sch 40 (3.91 mm wall) the ID is about 52.5 mm. Originally, nominal sizes approximated the bore of the standard-weight pipe of the day; as schedules multiplied, the OD was frozen for compatibility with fittings and the name stopped measuring anything.

Common confusion points

NPS 14 and above: OD in inches equals the NPS number (NPS 14 = 355.6 mm exactly). Below that, no such luck.
DN is not the ID and not the OD. Flanges, valves and fittings marked DN50 all fit the 60.3 mm pipe — that shared OD is the point of the system.
Flanges do not interchange across systems: a DN50 PN16 flange (EN 1092-1) and a 2 in Class 150 flange (ASME B16.5) fit the same pipe but have different bolt patterns and facings.
EN 10220 has Series 2 and 3 diameters (older national sizes) alongside Series 1; a DN100 pipe to a Series 2 row has a different OD. Piping built to Series 1 / ASME matches; always confirm the series on legacy European plant.
Threads are named nominally too: a 2 in BSP or NPT thread does not measure 2 inches across.

Working rule

Specify pipe by OD × wall in millimetres (or NPS/DN plus schedule) plus the dimensional standard. If a drawing says only "50 mm pipe", ask — three different products answer to that name.