**Units: **ft=foot, m=meter, s=second.

**Hazen-Williams Equation:** V = k C (D/4)^{0.63} S^{0.54}
where S = h_{f} / L and Q = V π D^{2} / 4

k is a unit conversion factor:

k=1.318 for English units (feet and seconds). k=0.85 for SI units (meters and
seconds)

C=Hazen-Williams Coefficient. D=Pipe inside diameter. π=3.14159...

Hazen-Williams Coefficient (C) varies from approximately C=60 for 40-year old cast iron pipe to C=150 for new plastic pipe. The higher the C, the smoother the pipe. Table of Hazen-Williams Coefficients.

The Hazen-Williams method is valid for water flowing at ordinary temperatures of 40 to 75 ^{o}F (4 to 25 ^{o}C)
through pressurized pipes. The Hazen-Williams equation is typically used to analyze city water supply systems.
For other liquids or gases, the Darcy-Weisbach
method should be used. Major loss (h_{f}) is the energy (or head) loss
(expressed in length units - think of it as energy per unit weight of fluid) due to
friction between the moving fluid and the pipe wall. It is also known as friction
loss. The Darcy-Weisbach method is generally considered more accurate than the
Hazen-Williams method. However, the Hazen-Williams method is very popular,
especially among civil engineers, since its friction coefficient (C) is not a function of
velocity or pipe diameter. Hazen-Williams is simpler than Darcy-Weisbach for
calculations where you are solving for flowrate (discharge), velocity, or diameter. More Discussion and References.

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