The sizes indicated in the metric systems are the actual size of the outside diameter of the pipe,in millimetres. Inch and metric systems (excepting 2 1/2” / 75 mm and 5” / 140 mm) are completely different sizes, and can only be joined together using adaptors or flanges with a common P.C.D.The following table indicates the approximate equivalent inch and metric sizes:Where applicable PVC fittings are according to ISO 727 and WRAS for potable water. Metric fittings with threads have DIN thread ie taper male parallel female. DIN 2999. Imperial Fittings conform to BS 4346/1 ISO 9002 for pipes made to BS 3505 and BS 3506. Thread are parallel BS 21 male and female.