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 ^oF (15.6 ^oC) with kinematic viscosity 1.13 cSt. For hot water with lower kinematic viscosity (0.55 cSt at 130 ^oF (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 ^oF (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)

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 (m³ /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/m², Pa)

γ = specific weight (N/m³ )

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

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm