DN vs NPS: the size conversion guide

DN is defined by ISO 6708 and used throughout the EN (European Norm) standards system. It is a dimensionless number that correlates to a nominal bore size in millimetres. DN100 corresponds to an approximate bore of 100 mm, but the actual bore depends on the pipe schedule and wall thickness — DN100 Schedule 40 and DN100 Schedule 80 have the same outside diameter but different wall thicknesses and therefore different bores.

NPS follows the same logic: NPS 4 (often written 4" or 4-inch) nominally corresponds to four-inch pipe, but the actual outside diameter is 114.3 mm regardless of schedule. For NPS 14 and above, the NPS number does equal the outside diameter in inches.

Both systems use nominal sizes, not actual bore measurements. The practical outcome is that a DN100 valve and an NPS 4 valve will both fit on the same pipe — but the flange drilling and face-to-face dimensions differ between EN 1092-1 and ASME B16.5 standards, so flanges are not interchangeable without an adapter or transition flange.

The table below gives the standard equivalents from DN15 (the smallest common valve size) through to DN1200. For sizes not in this table, NPS = DN × 0.03937 gives an approximate inch equivalent, but always verify against the manufacturer's dimensional drawing.

Even where DN and NPS sizes are equivalent in bore, EN 1092-1 and ASME B16.5 flanges are not dimensionally compatible. The flange OD, bolt circle diameter and bolt hole count differ between standards. Connecting an EN-flanged valve to an ASME-flanged pipe requires a purpose-made transition flange or adapter ring.

This matters most in projects that mix European and American equipment: for example, an EU-supplied pump with EN flanges connected to American-standard pipe. The issue should be resolved at the design stage by specifying a consistent flange standard throughout, or by specifying transition flanges at the interface.

JIS (Japanese Industrial Standard) flanges add a third incompatible flange system used on marine and Japanese-supplied equipment. JIS 5K, 10K and 16K refer to the pressure class, and dimensions again differ from both EN and ASME.

When ordering valves for a European project using EN piping, specify DN and PN (nominal pressure class). When ordering for a project using ASME piping, specify NPS and Class (e.g. Class 150). If the project mixes standards, identify the interface point and specify transition flanges explicitly.

On RFQs and data sheets, always state both the size standard and the flange standard — "DN100 EN 1092-1 PN16" is unambiguous; "4 inch" alone is not, because it could mean NPS 4 ASME B16.5 Class 150 or DN100 EN 1092-1 PN16 depending on the project context.