Hazen-Williams Formula
Head loss (ft) = 10.67 x L(ft) x Q^1.852(cfs) / (C^1.852 x D^4.87(ft)). Convert GPM to cfs: divide by 448.83. Convert head (ft) to PSI: multiply by 0.4335. Velocity (ft/s) = Q(cfs) / pipe area(ft^2).
Frequently Asked Questions
The Hazen-Williams equation is the most widely used empirical formula for friction head loss in water pipes: h_L = 10.67 x L x Q^1.852 / (C^1.852 x D^4.87), where h_L is head loss in feet, L is pipe length in feet, Q is flow rate in cubic feet per second, C is the Hazen-Williams coefficient, and D is pipe inside diameter in feet.
C-factor values: New copper or PVC = 140-150, New cast iron or ductile iron = 130, Concrete = 120-130, Galvanized steel = 120, Old cast iron (corroded) = 100-110, Very rough or corroded pipe = 80-100. Higher C values mean less friction and lower pressure loss.
PSI = head (feet) x 0.4335 (for water at typical temperatures). Conversely, 1 PSI = 2.31 feet of head. For example, 20 feet of head loss = 20 x 0.4335 = 8.67 PSI.
For residential water supply lines, a pressure drop of 2-5 PSI per 100 feet of equivalent pipe length is generally acceptable. Fire suppression systems have much stricter requirements defined by NFPA 13/14. Irrigation systems typically tolerate more pressure variation.
Each fitting (elbow, tee, valve) adds friction equivalent to some additional length of straight pipe. This is expressed as an equivalent length: a 90° elbow in 1-inch pipe adds approximately 2-3 feet of equivalent length. Add all fitting equivalents to the actual pipe length before calculating pressure loss.
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