The term Diameter Nominal refers to the internal diameter of a pipe. Together with the nominal pressure rating and the materials class, all dimensions of a piping line, e. g. flange dimensions, are defined by indicating the nominal diameter. Steel is often not specified, but assumed as material.
It is to be noted that in many cases the actual internal diameter deviates by several millimetres. It can only reliably be assumed that pipes from different manufacturer can be combined, if the nominal diameter DN is indicated with reference to the corresponding DIN standard.
The nominal diameter is specified according to EN ISO 6708 using the abbreviation DN (Diameter Nominal) followed by a dimensionless number corresponding to the approximate internal diameter in millimetres. For example, a DN 50 pipe denotes, according to EN 10255, a pipe with the external diameter of 60.3 mm and a wall thickness of 3.65 mm (resulting internal diameter 53 mm).