What is DN (Nominal Diameter)?

DN (Diamètre Nominal) is a dimensionless designator used in the metric system to describe the size of pipes and fittings per ISO 6708. It is roughly equivalent to the internal diameter in millimeters but does not correspond exactly to any measured dimension.

What is the difference between DN and NPS?

DN is the metric designation (ISO 6708), while NPS (Nominal Pipe Size) is the North American designation (ASME). They refer to the same pipe sizes but use different numbering: DN uses millimeters (DN50) while NPS uses inches (NPS 2). The actual outside diameters are identical.

Why is DN25 not equal to 25 mm?

DN is a nominal designator, not an actual dimension. DN25 corresponds to a 1-inch pipe with an actual OD of 33.4 mm. The DN number is loosely based on the approximate bore (internal diameter), but it is standardized and does not change with wall thickness.

Is DN the same as pipe OD?

No. DN is a nominal label, not a measurement. The actual outside diameter (OD) is standardized per ASME B36.10 / ISO 6708 and is different from the DN value. For example, DN100 has an OD of 114.3 mm, not 100 mm.

Free Engineering Tool

Nominal Bore Converter

Cross-reference DN (Nominal Diameter), NPS (Nominal Pipe Size), inch fraction, and actual outside diameter in mm. Select a size or search the full table.

ISO 6708 ASME B36.10 DN 6 – DN 600

Results

Outside Diameter (OD)

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DN (Nominal Diameter)

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NPS (Nominal Pipe Size)

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Inch Fraction

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OD in Inches

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DN	NPS	Inch Fraction	OD (mm)	OD (inch)

What is Nominal Bore (DN)?

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

What is NPS?

NPS (Nominal Pipe Size) is the North American equivalent designation, based on inches. NPS and DN describe the same pipe sizes with different numbering conventions. They share identical outside diameters per ASME B36.10.

Key Insight: The outside diameter (OD) of a pipe is fixed for a given DN/NPS. Only the wall thickness varies across different schedules (Sch 10, 40, 80, etc.), which changes the internal diameter and wall thickness.

Why DN25 ≠ 25 mm

DN values are historically based on approximate internal diameters, rounded and standardized. Over time, the actual OD was standardized to fixed values (inherited from the imperial system). DN25 corresponds to a 1-inch pipe with OD = 33.4 mm, not 25 mm.

DN is a label, not a measurement — always verify OD from the table

Conversion Pattern

For sizes DN350 and above, the DN value approximately equals the OD in mm:

DN350 → OD 355.6 mm (close to 350)
DN400 → OD 406.4 mm (close to 400)
DN500 → OD 508.0 mm (close to 500)

For smaller sizes, this pattern breaks down and table lookup is essential.

⚠️ Note: This table covers standard carbon steel pipe sizes per ASME B36.10 / ISO 6708. Stainless steel pipe (ASME B36.19) uses the same ODs but different wall thickness schedules (5S, 10S, 40S, 80S).

Nominal Bore Converter | DN ↔ NPS ↔ inch ↔ mm | ISO 6708

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026