How is the inside diameter of a pipe expressed?

The inside diameter of a pipe is most accurately expressed through the nominal pipe size. Nominal pipe size serves as an industry standard designation that provides a general indication of the pipe's diameter, although it does not directly correspond to the actual measurements. Nominal sizing streamlines communication in engineering and construction, aiding in the selection and specification of piping systems.

While nominal pipe size gives a good approximation, it is important to remember that actual inside diameters can vary based on the wall thickness of the pipe and the material from which it is made. Therefore, while nominal size serves as a convenient reference, for precise applications, the actual inside diameter should be determined, often through measurements or standard specifications.

The other options involve specific calculations or definitions that do not directly represent how the inside diameter is typically conveyed or understood in most piping systems.