How to measure the size of a copper water pipe?

[curiousorange]

I want to get the right size fitting for a copper water pipe. The house was built in the 80s I think. The pipe used to be connected to an immersion tank, so it is I think wider than pipes attached to taps.

Internet surfing suggests that pipes can vary in size by 1/8th of an inch if imperial, and then there are metric ones as well.

Is there any easy way to measure them? Should I measure the internal or external diameter? Would it be likely to be one particular size, from my description above?

Thanks

 

[kevplumb]

will be either 22mm or 3/4""

you can get adapters no problem

 

[Beerlover]

Imperial sizes refer to the internal diameter. metric the external.