Pipe size is standardized in North America as Nominal Pipe Size (NPS), but it is not always the actual measured size of the opening. To determine pipe size, measure the inside diameter (ID) for a female fitting, the outside diameter (OD) for a male fitting, and then refer to an NPS table. For unthreaded pipe, the measurement corresponds directly. For threaded pipe, round the measurement up to find the correct corresponding pipe size. For example, if a threaded pipe measures 7/8” in ID or OD, round up to the next size which is 1”, which corresponds in an NPS table to a pipe size of 3/4”. For unthreaded pipe, if it measures .84” in ID or OD, this corresponds directly to a pipe size of 1/2”, without rounding up.
Pipe dimensions for tees and wyes are represented as A x B x C. “A” and “B” indicate the main run on the fitting, and “C” indicates the branch.