When selecting copper fittings for plumbing and heating applications, the inside diameter of the copper pipe is the only dimension to take into consideration. Copper classified as 1 inch pipe, for example, has an inside diameter of 1 inch. This means that if you must connect two pieces of 1 inch pipe with an elbow, you should use a 1” x 1” elbow. The elbow does not measure exactly 1 inch, but it accepts 1 inch copper.
The inside diameter (ID) and outside diameter (OD) of a piece of copper (pipe or fittings) differ by 1/8”. Therefore the 1” pipe will have an OD of 1-1/8”. In order to fit over the pipe, the 1” x 1” elbow will have an ID of slightly over 1-1/8”. Its OD, as a result, would be 1-1/4".
Keep in mind that these rules only apply to copper used for plumbing and heating. In air conditioning and refrigeration situations, copper is sized by outside diameter.
