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DN (Diamètre Nominal) is a dimensionless designator used in the metric/ISO system to describe the size of pipes, fittings, flanges, and valves. Defined by ISO 6708, it is an alphanumeric label for reference purposes — it does not correspond exactly to any measured dimension of the pipe.

DN is roughly equivalent to the approximate internal diameter in millimeters, but the actual bore depends on wall thickness (schedule).

DN (ISO 6708) and NPS (ASME) refer to the same pipe sizes using different conventions:

• DN — metric designation, number loosely based on mm (e.g. DN50)
• NPS — North American designation, based on inches (e.g. NPS 2)

The outside diameters are identical. DN50 = NPS 2 = OD 60.3 mm. The choice depends on regional standards and project specifications.

DN is a nominal (name-only) designation. The values originated from approximate internal diameters but were standardized into fixed sizes long ago. DN25 corresponds to a 1-inch pipe with an actual OD of 33.4 mm.

For sizes DN350 and above, the DN value does approximately equal the OD in mm. Below that, always use the table lookup.

No. DN is a label, not a measurement. The actual outside diameter (OD) is a fixed dimension standardized per ASME B36.10 / ISO 6708, and it differs from the DN number for most sizes.

For example: DN100 → OD 114.3 mm (not 100 mm), DN50 → OD 60.3 mm (not 50 mm). Always refer to the conversion table for actual dimensions.