DN, Outside Diameter and Wall Thickness: Understanding Tube and Pipe Sizes

For a round tube, outside diameter is the diameter across its outer surface, inside diameter is the diameter of the bore, and wall thickness is the thickness of the tube wall. Tubing is commonly specified by outside diameter and wall thickness, but not all tubes, pipes, and fittings use this convention. Swagelok's tubing and pipe overview

In an OD × wall specification, the × separates the two dimensions rather than indicating multiplication. Both dimensions need units. Numbers on a drawing in millimeters cannot be compared directly with those in an inch-based catalog.

ISO 6708 defines DN as a nominal size designation. The number after DN is dimensionless, not a measured ID or OD. To find the corresponding dimensions, consult the applicable product standard. ISO 6708 public preview

Before using a DN-to-OD table, check its standard and product series. A relationship for one series is not a universal conversion for tubes, valves, and fittings. The DN number cannot simply be treated as a diameter in millimeters.

For a circular cross-section with concentric inner and outer surfaces and uniform wall thickness:

ID = OD − 2 × wall thickness

All three dimensions must refer to the same cross-section and use the same units. Under those conditions, increasing the wall thickness while holding OD constant reduces the bore.

The formula does not describe every manufactured cross-section. If a tube is oval, has an eccentric wall, or has been formed into a non-round shape, one OD and wall value cannot describe all its internal dimensions.

The OD and wall thickness on a drawing or specification may be nominal values. Not every tube or cross-section will measure exactly those values. To determine whether a tube meets the specification, compare its measurements with the tolerances on the drawing or in the product standard.

The tube body's OD also does not define the connection dimensions of a flared, reduced, or otherwise formed end. Identify where each dimension applies. On a shaped component, round tube ends and the formed section between them need separate dimensions; a single “tube diameter” cannot describe the whole part.

For measurement methods and checks of cross-sectional shape, see incoming tube dimensional inspection.

Two components marked with the same DN will not necessarily connect. Their end types, standards, and product series still need to match. For fittings selected by tube OD, matching the diameter is not the only requirement either.

Swagelok's tubing guidance, for example, covers material, hardness, surface condition, and wall thickness as well as diameter. Check the documentation for the fitting you are using to find its tubing requirements. OD alone is not enough. Swagelok Tubing Data

In liquid-cooling pipework, custom metal tube sections and standard fittings may also use different size conventions. Specify the tube-body dimensions separately from the end connections. For how these components are supplied together, see standard fittings and custom liquid-cooling tube components.

Common specification questions