In a piping schematic, the size of each pipe is indicated by its:

In a piping schematic, the size of each pipe is most accurately indicated by its inside diameter. This measurement is crucial because it directly affects the flow capacity and pressure within the system. The inside diameter provides a clear understanding of how much fluid can pass through the pipe and is essential when calculating flow rates, pressure drops, and overall system performance.

While outside diameter and material type provide useful information about the physical characteristics and construction of the piping, they do not give an accurate representation of the effective carrying capacity of the pipe. Length determines how much pipe is needed but does not relate to pipe sizing in terms of its flow characteristics. Therefore, using inside diameter as the standard for indicating pipe size is critical for proper design and functionality in any piping system.