How do I know what size copper pipe I have?
The nominal size of copper pipe, whether supply lines or DWV, is always 1/8 inch less than the OD measurement. For example, copper pipe that measures 5/8 inch OD, is called 1/2- inch pipe. The ID varies because wall thickness varies. Type K has the thickest wall, followed by Type L, Type M and DWV. In the plumbing trade the size of copper tubing is measured by its outside diameter in millimetres.