Which factor is critical when measuring the size of a pipe fitting?

• A. Wall thickness
• B. Inner diameter
• C. Outer diameter
• D. Pipe length

Answer: (C)

The critical factor when measuring the size of a pipe fitting is the outer diameter. Outer diameter is essential because it determines how the fitting will interface with other pipes and fittings in a plumbing or piping system. When joining pipes or fittings, having the correct outer diameter ensures that connections can be made securely without leaks and that the fittings fit properly into the intended system.

While other factors like wall thickness, inner diameter, and pipe length are important in their contexts, they do not directly dictate how a fitting connects with pipes in the same way that outer diameter does. Wall thickness can affect the strength and pressure rating of a fitting, inner diameter is relevant for flow capacity, and pipe length is important for determining overall system layout, but the outer diameter is the most crucial measurement for fitting compatibility and installation.