The nominal diameter is referred to as the pipe diameter or the mating dimension of a valve (with valves this is usually referred to as the nominal size). In combination with the nominal pressure, the approximate inside diameter is specified in millimetres to ISO 6708 in the form of a dimensionless figure, preceded by the designation DN (DN 100).
The table "Nominal diameters to DIN 2402" provides an overview. It does not specify numerical values. See Fig. 1 DN