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

Darcy-Weisbach Friction Loss Equation:

If non-circular duct, D computed from D = 4A/P

D is called the "duct diameter" to keep the terminology general to include circular pipes and non-circular pipes, also known as ducts. For rectangular pipes (ducts), D=4A/P is known as the hydraulic diameter. More information about hydraulic diameter can be found on our non-circular to circular pipe conversion page.

g = acceleration due to gravity = 32.174 ft/s^{2} = 9.806 m/s^{2}.

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
duct. It is also known as friction loss. The Darcy-Weisbach method is
generally considered more accurate than the Hazen-Williams
method. Additionally, the Darcy-Weisbach method is valid for any liquid or gas;
Hazen-Williams is only valid for water at ordinary temperatures (40 to 75 ^{o}F).
The Hazen-Williams method is very popular, especially among civil engineers, since its
friction coefficient (C) is not a function of velocity or duct diameter.
Hazen-Williams is simpler than Darcy-Weisbach for calculations where you are solving for
flowrate, velocity, or diameter. More Discussion and
References.

The Darcy-Weisbach calculation does not check for unreasonable inputs such as negative values. All values should be entered as positive.

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