Plumbers and pipefitters determine pipe size based on which dimension?

• A. Wall thickness
• B. Nominal size
• C. Inside diameter
• D. Outside diameter

Answer: (C)

The size of a pipe is determined by the space inside it—the bore. That inside diameter sets how much water can flow and how fittings, valves, and fixtures will mate with the pipe. The label used to designate pipe size is a nominal size, which is a standardized designation that roughly reflects the bore, but the actual inside diameter depends on the material and wall thickness (the schedule). The outside diameter can vary with wall thickness and does not tell you the flow capacity. So, when sizing and selecting pipe and fittings, the relevant dimension is the inside diameter.