How this calculator works
Hazen-Williams is an empirical water-pipe relation in which a larger C means less friction. This SI version uses metres and cubic metres per second. Material presets select representative values within EPA new-pipe ranges; they do not account for ageing, deposits or condition. The pressure conversion uses entered density and gravity. Fittings, valves and elevation are excluded. For other liquids, large temperature changes or laminar flow, use Darcy-Weisbach instead.
Formula
hf = 10.67 L Q^1.852/(C^1.852 D^4.871), using SI inputs. Delta p = rho g hf. Mean velocity v = 4Q/(pi D^2).
Worked example
For a 100 m pipe with D = 0.1 m, C = 130 and Q = 0.01 m3/s, head loss is about 1.90608 m. Doubling length doubles head loss; doubling flow multiplies it by about 3.61.
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Frequently asked questions
Does the coefficient describe wall roughness height?
No. Hazen-Williams C is an empirical coefficient, whereas Darcy-Weisbach absolute roughness is a length.
Can I use a new-pipe value for an old pipe?
Only as an assumption. Deposits and deterioration can reduce the effective bore and change the coefficient substantially.
Is static lift included?
No. This is friction along the pipe. Add elevation and local component losses in a full system calculation.