FYI, copper pipe and copper tubing both are controlled at the OD. Ditto with steel pipe and tubing. Tubing OD (e.g., 0.25") is the same as nominal size (1/4"). That's not true with pipe, where the iD is closer to nominal size. 1/2" pipe is closer to 5/8" OD. But think about it. Both tubing and pipe go into fitting that have to be able to accommodate a consistent OD. Heavier wall tubing and pipe therefore has a smaller ID than standard wall size.