In Australia we need to deal with joining both metric and inch piping systems; so we need to convert and understand how it measures up. For those unfamiliar with the difference between metric and inch sizes the following note may be helpful. In imperial systems, the sizes of pipes, fittings and other components such as valves are identified by reference to the nominal size of the bore of the pipe expressed in inches and fractions of an inch.
In metric systems, however, sizes are identified by references to the outside diameter of the pipe expressed in millimetres. For converting metric to imperial it should be understood that metric sizes are not simply inch sizes which have been converted into millimetres and called metric; their actual dimensions are slightly different and they are with the exception of 2½ ” (75 mm) and 5″ (140 mm) not interchangeable.