The Hazen-Williams formula is an empirical rule, that holds well for cold water running in pipes under turbulent flow conditions. This is very suitable for situations such as domestic piping and hosing, sprinkler and irrigation systems, etc. For gravitational flow, and for open-channel flow, other calcs are available.
Typical values of the roughness (friction loss) coefficient include: 100 (concrete, cast iron); 120 (steel); 140 (cement); 150 (copper, plastics).