DN vs NPS Valve Size: The Nominal Designations Behind Metric and Inch Piping

The Short Answer

DN and NPS are two ways of naming valve and pipe sizes. DN (diameter nominal) is the metric designation — DN15, DN25, DN100 — used in European and international standards (ISO 6708, EN 1092 flange series). NPS (nominal pipe size) is the inch-based designation — NPS 1/2, NPS 1, NPS 4 — used in US standards (ASME B36.10/B36.19 pipe, ASME B16.5 flanges). The numbers correspond: DN15 = NPS 1/2, DN25 = NPS 1, DN100 = NPS 4, and so on. Neither DN nor NPS is an actual outside diameter; both are nominal designations that match to standard pipe dimensions, and the actual outside diameter differs from both the number and from pipe to pipe wall schedule. The conversion table is fixed for the common sizes, but the real engineering content is in the flange and wall-thickness systems behind the numbers.

What DN Means

DN is defined in ISO 6708 as the nominal size designation, expressed in millimeters, common to components in a piping system. It is a round number close to the outside diameter in millimeters for small sizes, but it is not the outside diameter — DN100 pipe has an outside diameter of 114.3 mm (4 in), not 100 mm. DN is the basis of the European flange system: EN 1092-1 flange dimensions are keyed to DN, and the same DN across a pipe system connects pipe, flanges, valves, and fittings of the same nominal size. Because DN is a pure designation, two components with the same DN are not automatically interchangeable unless their standards and pressure classes also match — the same compatibility rule that governs ASME B16.5 vs B16.47 flanges on the US side.

What NPS Means

NPS is the US nominal pipe size designation, expressed in inches, defined through ASME B36.10 (welded and seamless wrought steel pipe) and B36.19 (stainless steel pipe). The designation is an inch-based nominal size: NPS 1/2 through NPS 12 use fractions or integers that are close to the actual outside diameter in inches only for sizes above NPS 12; below NPS 12 the outside diameter is larger than the nominal size (for example, NPS 1 has an outside diameter of 1.315 in, and NPS 6 has 6.625 in). For NPS 14 and above, the NPS number equals the outside diameter in inches. The wall thickness is specified separately by schedule numbers (Sch 40, Sch 80), which is why NPS alone does not define the pipe — the schedule is part of the designation in practice.

DN and NPS Correspondence

DN (metric)	NPS (inch)	Outside Diameter (mm / in)	Note
DN15	NPS 1/2	21.3 mm / 0.840 in	Common pairing
DN20	NPS 3/4	26.7 mm / 1.050 in	Common pairing
DN25	NPS 1	33.4 mm / 1.315 in	Common pairing
DN40	NPS 1 1/2	48.3 mm / 1.900 in	Common pairing
DN50	NPS 2	60.3 mm / 2.375 in	Common pairing
DN80	NPS 3	88.9 mm / 3.500 in	Common pairing
DN100	NPS 4	114.3 mm / 4.500 in	Common pairing
DN150	NPS 6	168.3 mm / 6.625 in	Common pairing
DN200	NPS 8	219.1 mm / 8.625 in	Common pairing
DN300	NPS 12	323.9 mm / 12.750 in	Common pairing

The DN-NPS pairing follows ISO 6708 and ASME pipe dimension tables and is fixed for the common sizes; confirm against the current standard editions for unusual sizes (data needs further verification for your exact size).

Application Example: A Metric Valve on an Inch Pipeline

A European fabricator supplied DN150 flanged ball valves for a US-designed skid where the line was specified as NPS 6, Class 300, ASME B16.5. The outside diameter matched (168.3 mm), but the flange face-to-face length, bolt holes, and rating followed EN 1092-1 PN16 — incompatible with the B16.5 Class 300 flanges on the skid. The mismatch was caught during drawing review, and the valves were re-specified to ASME B16.34/B16.5 with NPS 6 sizing. The lesson: DN and NPS agree on the nominal size, but the flange standard, pressure class, and face-to-face series decide whether the parts fit — exactly the cross-system reality explained in PN vs Class valve pressure rating, where the two systems also look similar on paper but differ in practice.

Common Misconceptions

The first misconception is that DN is the outside diameter in millimeters — DN100 is not 100 mm outside diameter. The second is that NPS 12 and below equals the outside diameter in inches — the outside diameter is larger than the number below NPS 12. The third is that converting DN to NPS is all that is needed for a fit — the flange standard, rating, and face-to-face must also match. The fourth is that “DN 4” and “NPS 4” are the same name in different units — they are the same nominal size, but each is defined in its own standard system. The fifth is ignoring the wall-thickness system: the same NPS comes in many schedules, and the schedule, not the size, often decides what the valve connects to.

Practical Recommendation

Specify the full designation, not just the size number: “DN100, EN 1092-1, PN16” or “NPS 4, ASME B16.5, Class 300.” When a datasheet arrives in the other system, convert the nominal size, then check the flange standard, rating, and face-to-face length against your piping specification before accepting the valve. For cross-system projects, name one governing size system (typically the one used for the piping) and require all components to follow it.

Conclusion

DN and NPS are the same nominal sizing concept expressed in metric and inch designations, with a fixed correspondence for common sizes. The engineering is not in the conversion — it is in the flange standard, pressure class, and wall schedule behind the number. Specify the full designation and the two systems stop being a source of wrong valves.

This article provides general educational knowledge only and is not professional engineering advice.

Sources & Further Reading

ISO, ASME, EN

Tags: Valve Standards