Plumbing fittings from good plumbing suppliers are available in metric and imperial sizes.

You need to use either imperial or metric all the way through, but there are some copper fittings that are available that are conversion fittings that will convert from imperial to metric...

These fittings are usually compression or solder fittings.

And, yes, I have faced the same issue on my parent's house with old imperial sized pipes and I noe keep the conversion fittings to convert to the metric sizes ie 1/2" pipe is now 15mm etc.

Plumbing fittings from good plumbing suppliers are available in metric and imperial sizes.

You need to use either imperial or metric all the way through, but there are some copper fittings that are available that are conversion fittings that will convert from imperial to metric...

These fittings are usually compression or solder fittings.

And, yes, I have faced the same issue on my parent's house with old imperial sized pipes and I now keep the conversion fittings to convert to the metric sizes ie 1/2" pipe is now 15mm etc.