3/4" screwed copper would be 27mm od and the wall thickness 3mm in new money.
1/2" screwed copper would be 20mm.
7/8" on my tape measure is 22mm not 25mm
1/2" screwed copper would be 20mm.
but I now find it is 7/8" (or ~ 25mm)
7/8" on my tape measure is 22mm not 25mm