What does the Moody chart relate?

• A. It plots f versus Re for various ε/D; shows how f changes with flow regime and pipe roughness
• B. It plots Re versus f for various ε/D
• C. It plots head loss versus diameter
• D. It plots velocity versus Reynolds number for laminar flow

Answer: (C)

The Moody chart is a map of how the friction factor f, used in the Darcy–Weisbach equation, varies with Reynolds number Re for different pipe roughness levels ε/D. On the chart, f is on the vertical axis and Re on the horizontal axis, with separate curves for each relative roughness. This lets you read off the friction factor once you know the flow regime (via Re) and the pipe’s roughness, and then plug f into h_f = f (L/D) (V^2/2g) to find head loss. In laminar flow, f = 64/Re, so the relation is straightforward; in turbulent flow, f depends on both Re and roughness, and the curves reflect that. The chart does not plot head loss directly against diameter, nor does it plot velocity against Reynolds number for laminar flow—its purpose is to relate f, Re, and ε/D.