 From experimental measurements, the general features of the variation of "f""D" are, for fixed "relative roughness" ε / "D" and for Reynolds number Re = "V" "D" / ν > ~2000,

The experimentally measured values of "f""D" are fit to reasonable accuracy by the (recursive) Colebrook–White equation, depicted graphically in the Moody chart which plots friction factor "f""D" versus Reynolds number Re for selected values of relative roughness ε / "D".In a design problem, one may select pipe for a particular hydraulic slope "S" based on the candidate pipe's diameter "D" and its roughness ε.With these quantities as inputs, the friction factor "f""D" can be expressed in closed form in the Colebrook–White equation or other fitting function, and the flow volume "Q" and flow velocity "V" can be calculated therefrom.