The inside diameter of a pipe is expressed as the:

The inside diameter of a pipe is commonly referred to as the nominal pipe size. This is a standardized designation that provides a general idea of the size of the pipe for ease of communication and identification in the industry. The nominal pipe size does not necessarily equate to the actual physical measurements, such as the inside or outside diameter, but it helps users understand the flow characteristics and compatibility with fittings and other components.

Using nominal sizes allows for uniformity across various materials and applications in the plumbing and HVAC sectors. It's essential for ensuring that systems are designed and installed correctly, as it simplifies specification and ordering processes.

In contrast, the other choices do not accurately represent how the inside diameter is expressed. The subtraction of wall thickness from the outside diameter provides a means to calculate the inside diameter, but it is not the terminology used to denote it. The formula mentioned does not relate to measuring pipe size in this context, and adding wall roughness to pipe size does not pertain to the expression of inside diameter either.