IS0 6708: 1995 pdf download – Pipework components – Definition and selection of DN (nominal size).
1 Scope This International Standard gives the definition of DN (nominal size) when applied to components of a pipework system, as specified in those standards which use the DN designation system. NOTE 1 Other methods of size designation are also used, e.g. products having threaded, compression, solder or welded end connections or products designated by NPS (nominal pipe size), OD (outside diameter), ID (inside diam- eter), but these are not defined in this International Stan- dard. This international Standard gives a list of preferred DN values. 2 Definition 2.1 DN: An alphanumeric designation of size for components of a pipework system, which is used for reference purposes. It comprises the letters DN fol- lowed by a dimensionless whole number which is in- directly related to the physical size, in millimetres, of the bore or outside diameter of the end connections. NOTES 2 The number following the letters DN does not represent a measurable value and should not be used for calculation purposes except where specified in the relevant standard.
3 In those standards which use the DN designation sys- tem, any relationship between DN and component dimen- sions should be given, e.g. DN/OD or DN/ID.