Before revision: SI units When used to calculate the head loss with the International System of Units, the equation becomes: where: S = Hydraulic slope hf = head loss in meters (water) over the length of pipe L = length of pipe in meters Q = volumetric flow rate, m3/s (cubic meters per second) C = pipe roughness coefficient d'' = inside pipe diameter, m (meters) Note: pressure drop can be computed from head loss as hf × the unit weight of water (e.g., 9810 N/m3 at 4 deg C) See also Darcy–Weisbach equation and Prony equation for alternatives Fluid dynamics Friction Minor losses in pipe flow Plumbing Pressure Volumetric flow rate References Further reading Williams and Hazen, Second edition, 1909 External links Engineering Toolbox reference Engineering toolbox Hazen–Williams coefficients Online Hazen–Williams calculator for gravity-fed pipes.
After revision: SI units When used to calculate the head loss with the International System of Units, the equation will then become where: S = Hydraulic slope hf = head loss in meters (water) over the length of pipe L = length of pipe in meters Q = volumetric flow rate, m3/s (cubic meters per second) C = pipe roughness coefficient d'' = inside pipe diameter, m (meters) Note: pressure drop can be computed from head loss as hf × the unit weight of water (e.g., 9810 N/m3 at 4 deg C) See also Darcy–Weisbach equation and Prony equation for alternatives Fluid dynamics Friction Minor losses in pipe flow Plumbing Pressure Volumetric flow rate References Further reading Williams and Hazen, Second edition, 1909 External links Engineering Toolbox reference Engineering toolbox Hazen–Williams coefficients Online Hazen–Williams calculator for gravity-fed pipes.