In the American plumbing industry, the size of copper tubing is designated by its nominal diameter, which is 1⁄8th inch less than the outside diameter. This nominal diameter does not match any other physical dimension on the pipe; it may be larger or smaller than the inside diameter depending on the size of pipe. The American refrigeration industry uses different copper pipe called ACR (air conditioning and refrigeration field services) pipe, which is sized directly by its outside diameter (OD) and a type letter indicating wall thickness. Therefore, 1 inch nominal type L copper tube and 1 1⁄8th inch type D ACR tube are exactly the size with different size designations. ACR pipe is manufactured without processing oils that would be incompatible with the oils used to lubricate the compressors in the AC system.
Except for this difference between ACR (types A and D) and plumbing (types K, L, M and DWV) pipes, the type only indicates wall thickness and does not affect the outside diameter of the tube. Type K 1⁄2 inch, type L 1⁄2 inch, and type D 5⁄8 inch ACR all have the same outside diameter of 5⁄8 inch.