The Hazen–Williams equation is an empirical formula which calculates pressure drop of water in circular pipes caused by friction. It is used in the design of water pipe systems such as fire sprinkler systems, water supply networks, and irrigation systems.

The Hazen–Williams equation has the advantage that the coefficient C is not a function of the Reynolds number, but it has the disadvantage that it is only valid for water.
Input parameters are flow, pipe diameter, lenght of pipe and C is a roughness coefficient, known as William Hazens coefficient, with values ​​between 90 (more rough pipes) and 140 (smooth pipes, ex PVC).

You can select between imperial (U.S. customary units) and metric unit (International System of Units).

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