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Irish Copper Pipe vs Metric Copper Pipe

This is an area where things are not as simple as they may seem! Unfortunately for the average DIY enthusiast or anyone not very familiar with plumbing, pipe sizes in Ireland (and also the connections) can be confusing.

In the 1970’s Britain changed from Imperial plumbing sizes to metric plumbing sizes. However, Ireland did not change to metric sizes and still uses the Imperial system.

If we take two of the most common Imperial plumbing sizes, 1/2″ and 3/4″. They changed to metric 15 mm and 22 mm respectively in Britain. Now this seems straight forward enough but there is a catch. The Imperial sizes are based on the internal bore diameter of the pipe and the metric sizes are based on the external diameter of the pipe.

Internal Diameter (ID) is used for Imperial and Outside Diameter (OD) is used for metric

Irish Imperial sizes for copper pipes are as below:

1/2″ Irish Pipe is as below:
1/2″ bore is 12.7 mm. If the thickness of the pipe is added on it comes to 14.7 mm which is not quite 15 mm.

3/4″ Irish Pipe is as below:
3/4″ bore is 19.05 mm. If the thickness of the pipe is added on it comes to 21 mm which is not 22 mm.

This leads to plumbers using adapters to go from different size pipes if they end up on a job with Irish Imperial and British Metric pipe.

Thread Sizes

The pipe sizes above are not to be confused with thread sizes. In plumbing in Ireland and Britain (and other countries) the thread standard used is BSP or British Standard Pipe. In the USA however, you will see NPT (National Pipe Thread) is the standard. BSP thread size nominal names were originally based on the inner diameter (measured in inches) of the tube for which the thread was intended. Now most tubes have thinner walls to save cost, so the inner diameter is often bigger than the nominal size. Please see the diagram below for examples of the outer diameter of the various BSP threads:

As you can see, 1/2″ thread ( 1/2″ = 12.7 mm) has a outside diameter of 20.995 mm.

Thread Sizes for Different Applications

Typical uses and sizes are:

1/4″ – Compressed air connections
3/8″ – Oil line connections

1/2″ – Basin and Kitchen sink taps, radiator inlets, old outside tap outlets
5/8″ – Old outside tap outlets (very rare)
3/4″ – Bath taps, washing machine inlet hose connections, modern outside tap outlets
1″ Domestic heating boilers
1.25″ – Wash hand basin wastes (bathroom sink)
1.5″ – Kitchen sink wastes (kitchen sink), bath wastes, WC siphon outlets
2″ – Modern toilet flush valve outlets

At the our taps come in 1/2″ BSP (Basin and Kitchen) and 3/4″ BSP (Bath taps). Our wastes are 1.25″ for Basin Wastes and 1.5″ for Kitchen Wastes.

See here for details of our taps and waste collection.


  • I will purchase a German built hot water heater. Would you know where I can get a couple 1/2″ adapters? I live in the USA which is NPT. I need adapters that are NPT on one end and DIN German on the other. Your help would be appreciated immensely. Thank you very much David Winters.

  • Lucy says:

    I need to connect four lengths of Irish 1/2″ copper pipe to make a square for a ( non plumbing related) project.
    Based on your very helpful guide and explanations above, could you please confirm that I will need 15mm copper elbow connectors?
    They need to be a tight enough fit to prevent the pipes from sliding out if accidentally knocked, but not so secure to be non de-mountable either incase I need to dismantle for storage.

  • Robert says:

    I am renovating a house in France. I want to bring over Irish taps, shower valve etc. The French copper is a different size to Irish copper, Do you have or know who might have a solution for this/.
    Thank you

  • Kevin Walsh says:

    Will 76mm copper fitting fit 3 inch imperial copper pipe

  • Dorota says:

    Where can I get an adapter for old Irish copper pipe to the metric fittings? Basically old pipes are coming out and I need to install the water heater on it and everything what I plugged is leaking due to the difference in the “15 mm” Irish and 15 mm metric

  • david Freitag says:

    I’m an American building a house in County Kerry. Ae the drains and thread connections similar in the USA and Ireland?

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