What Is NPS? Nominal Pipe Size Explained

Quick Answer

NPS (Nominal Pipe Size) is a dimensionless designator that identifies pipe by a number that does not directly equal the pipe’s outside diameter or inside diameter. For NPS 1/8 through NPS 12, the NPS number is an approximate bore size. From NPS 14 onward, NPS equals the actual OD in inches.

NPS is the North American standard for designating pipe size, defined in ASME B36.10 (carbon and alloy steel) and ASME B36.19 (stainless steel). The metric equivalent is DN (Diametre Nominal), used in Europe and defined in ISO 6708 and EN standards.

NPS to Actual OD

The relationship between NPS and actual outside diameter is not intuitive for small sizes. The OD is fixed for each NPS regardless of wall thickness or schedule.

NPS	DN	Actual OD (mm)	Actual OD (in)
1/2	15	21.3	0.840
3/4	20	26.7	1.050
1	25	33.4	1.315
1-1/2	40	48.3	1.900
2	50	60.3	2.375
3	80	88.9	3.500
4	100	114.3	4.500
6	150	168.3	6.625
8	200	219.1	8.625
10	250	273.1	10.750
12	300	323.9	12.750
14	350	355.6	14.000
16	400	406.4	16.000
24	600	610.0	24.000

Notice that NPS 4 pipe has an OD of 4.500 inches, not 4.000 inches. The NPS value approximated the internal diameter of early iron pipes with standard wall thickness. As the system evolved, the OD was standardized and the ID became a function of wall thickness (schedule).

NPS vs DN Conversion

DN (Diametre Nominal) is the metric counterpart to NPS, defined in millimeters. The conversion is standardized but not a simple multiplication:

• NPS 1 = DN 25 (but the OD is 33.4 mm, not 25 mm)
• NPS 2 = DN 50 (but the OD is 60.3 mm, not 50 mm)
• NPS 14 = DN 350 (OD is 355.6 mm)

Both NPS and DN are labels, not measurements. The actual OD is identical regardless of whether NPS or DN is used.

Why NPS Matters

Every piping component, including flanges, fittings, and valves, is designated by NPS to ensure dimensional compatibility. An NPS 6 flange mates with an NPS 6 pipe and an NPS 6 fitting because they all share the same OD of 168.3 mm.

Pro Tip

When specifying pipe for international projects, always include both NPS and DN designations on purchase orders and piping isometrics. This avoids confusion between North American and European suppliers.

NPS combined with schedule fully defines a pipe’s dimensions: the NPS sets the OD, and the schedule sets the wall thickness.

Read the full guide to pipe types