Darcy Friction Factor Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Last verified 25 June 2026.

The Darcy friction factor sets how much pressure a fluid loses to friction as it flows through a pipe, and it is the key term in the Darcy-Weisbach equation used to size pipework and pumps. Its value depends on the Reynolds number, which compares inertial to viscous forces, and on the relative roughness of the pipe wall. In laminar flow, below a Reynolds number of about 2,300, the factor is simply sixty-four divided by the Reynolds number and roughness plays no role. In turbulent flow it is found from the Colebrook equation, an implicit relation solved here by iteration until it converges. This calculator takes the Reynolds number and the relative roughness, decides which regime applies, and returns the Darcy friction factor to four decimal places. The Colebrook solution typically converges in a handful of iterations to a stable value. The approach is the standard textbook method for pipe friction. Fluid mechanics conventions and related engineering standards are published by US federal agencies including the National Highway Traffic Safety Administration. Every figure is computed deterministically from these established relations, shown below, with a worked example that reconciles exactly to the calculator so you can check each step for yourself.