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Hazen-Williams Pressure Loss Equation

The Hazen-Williams equation can be used to calculate the pressure drop (psi) or friction loss in pipes or tubes.

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Hazen-Williams Formula in Imperial Units

The Hazen-Williams 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)


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 Hazen-Williams 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 oC) with kinematic viscosity 1.13 cSt. For hot water with lower kinematic viscosity (0.55 cSt at 130 o F (54.4 oC)) 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 oC) , the Darcy Weisbach equation should be used for other liquids or gases.

Online Hazens-Williams 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 c-value for cast iron and wrought iron pipes or tubes ranges from 80 to 150, with average value 130 and design value 100 .
  • The c-value for copper, glass or brass pipes or tubes ranges from 120 to 150 , with average value 140 and design value 140 .
  • The c-value for cement lined steel or iron pipes has average value of 150 and design value 140 .
  • The c-value for epoxy and vinyl ester pipes can be set to 150 .

Hazen-Williams Formula in Metric Units

h = 10.67  q 1.85 / (c 1.85 d h 4.8655 )                                       (2)


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


p = pressure loss (N/m2, Pa)

γ = specific weight (N/m 3 )

Specific weight of water at 4 oC is 9810 N/m 3 .

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