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Metric Sizes in Household Plumbing

If you live in an old house matching new plumbing to old can be a problem since old pipes and fittings will be imperial size, and all new copper pipe is only available in metric sizes. To solve the problem you will need to use special adapters when the sizes do not match. A good range of these adapters are available in DIY stores.

Note that the sizes of metric pipes refer to the outside diameters, but those of imperial pipes refer to the bores (the diameter of the hole).

Capillary plumbing fittings

For integral-ring capillary fittings (where the solder is pre-loaded into the fitting) you'll always need to use special adaptors to join metric size copper pipe to imperial size pipe (these adaptors are just ordinary fittings with one end slightly larger than the other). End-feed capillary fittings can be made to work simply by adding more solder - but it does require more skill to do this.

Compression plumbing joints

For compression joints, the 12mm, 15mm, 28mm and 54mm fittings will work with 3/8in, l/2in, 1 in and 2in pipe. For other imperial pipe sizes, you'll need to use special adaptors.