[LazyPup]

There is no 5/8 galvanized iron pipe!

You are mistakenly measuring the outside diameter while all "Pipe" is specified by inside diameter.

Galvanized iron pipe used in the potable water distribution system for residential applications are 1/2", 3/4" 1" and in rare circumstances 1-1/4". 1-1/2" & 2".

When replacing galvanized iron pipe with copper pipe you must use the same size of pipe or the entire potable water distribution system would need to be recalculated for load.

Also, you must use a dielectric union, dielectric nipple or a 6" hardened bronze nipple to make a transition from galvanized iron to copper pipe.