HazenWilliams Pressure Loss Equation
The HazenWilliams equation can be used to calculate the pressure drop (psi) or friction loss in pipes or tubes.
HazenWilliams Formula in Imperial Units
The HazenWilliams equation for calculating head loss in pipes and tubes due to friction can be expressed as:
P _{ d } = 4.52 q ^{ 1.85 } / (c ^{ 1.85 } d _{ h } ^{ 4.8655 } ) (1)
where
P _{ d } = pressure drop (psi/ft pipe)
c = design coefficient determined for the type of pipe or tube  the higher the factor, the smoother the pipe or tube
q = flow rate (gpm)
d _{ h } = inside hydraulic diameter (inches)
Note! The HazenWilliams equation estimates an accurate head loss due to friction for fluids with a kinematic viscosity of approximately 1.1 cSt. More about fluids and kinematic viscosity .
The results is acceptable for cold water at 60 ^{ o } F (15.6 ^{o}C) with kinematic viscosity 1.13 cSt. For hot water with lower kinematic viscosity (0.55 cSt at 130 ^{ o } F (54.4 ^{o}C)) the error will be significant.
Since the Hazen Williams method is only valid for water flowing at ordinary temperatures between 40 to 75 ^{ o } F (4  14 ^{o}C) , the Darcy Weisbach equation should be used for other liquids or gases.
Online HazensWilliams Calculator  Imperial Units
The calculator below can used to calculate head loss in imperial units:
The Design Factor  c
The design factor is determined for the type of pipe or tube used:
 The cvalue for cast iron and wrought iron pipes or tubes ranges from 80 to 150, with average value 130 and design value 100 .
 The cvalue for copper, glass or brass pipes or tubes ranges from 120 to 150 , with average value 140 and design value 140 .
 The cvalue for cement lined steel or iron pipes has average value of 150 and design value 140 .
 The cvalue for epoxy and vinyl ester pipes can be set to 150 .
HazenWilliams Formula in Metric Units
h = 10.67 q ^{ 1.85 } / (c ^{ 1.85 } d _{ h } ^{ 4.8655 } ) (2)
where
_{ h } = head loss per unit pipe (m _{ h2o } /m pipe)
c = design coefficient determined for the type of pipe or tube  the higher the factor, the smoother the pipe or tube
q = flow rate (m ^{ 3 } /s)
d _{ h } = inside hydraulic diameter (m)
Pressure drop in Pa can be calculated from the head loss by multiplying the head loss with the specific weight of water:
p = h γ
where
p = pressure loss (N/m^{2}, Pa)
γ = specific weight (N/m ^{ 3 } )
Specific weight of water at 4 ^{o}C is 9810 N/m ^{ 3 } .
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