The friction factor in fully developed turbulent pipe flow depends on the Reynolds number and the relative roughness ε⁄D, which is the ratio of the mean height of roughness of the pipe to the pipe diameter. The functional form of this dependence cannot be obtained from a theoretical analysis, and all available results are obtained from painstaking experiments using artificially roughened surfaces (usually by gluing sand grains of a known size on the inner surfaces of the pipes).

The experimental results obtained are presented in tabular, graphical, and functional forms obtained by curve-fitting experimental data. In 1939, Cyril F. Colebrook (1910–1997) combined the available data for transition and turbulent flow in smooth as well as rough pipes into the following implicit relation known as the Colebrook equation:

$\frac{1}{\sqrt{f}}=-2.0log(\frac{ε⁄D}{3.7}+\frac{2.51}{Re\sqrt{f}})$

We make the following observations from the Moody chart:

For laminar flow, the friction factor decreases with increasing Reynolds number, and it is independent of surface roughness.

The friction factor is a minimum for a smooth pipe (but still not zero because of the no-slip condition) and increases with roughness

The transition region from the laminar to turbulent regime (2300< Re< 4000) is indicated by the shaded area in the Moody chart.

At very large Reynolds numbers (to the right of the dashed line on the chart) the friction factor curves corresponding to specified relative roughness curves are nearly horizontal, and thus the friction factors are independent of the Reynolds number.