# Hazen-Williams Pressure Loss Equation

### 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)

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 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 ^{o}C) * with kinematic viscosity

*1.13 cSt.*For hot water with lower kinematic viscosity

*(0.55 cSt at 130*the error will be significant.

^{ o }F (54.4^{o}C))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 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)

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 }.

## Related Topics

### • Fluid Flow and Pressure Loss

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