Colebrook Equation
Friction loss coefficients in pipes, tubes and ducts.
The friction coefficients used when calculating resistance or pressure loss (or major loss) in ducts, tubes or pipes can be calculated with the Colebrook equation
1 / λ^{1/2} = 2 log [ 2.51 / (Re λ^{1/2}) + (k / d_{h}) / 3.72 ] (1)
where
λ = DarcyWeisbach friction coefficient
Re = Reynolds Number
k = roughness of duct, pipe or tube surface (m, ft)
d_{h} = hydraulic diameter (m, ft)
The Colebrook equation is only valid at turbulent flow conditions.
Note!  the friction coefficient is involved on both sides of the equation. The equation must therefore be solved by iteration.
The Colebrook equation is generic and can be used to calculate friction coefficients for different kinds of fluid flows  like air ventilation ducts, pipes and tubes with water or oil, compressed air and much more.
Colebrook Online Calculator
The friction coefficient can be calculated with the online Colebrook calculator below.
The default values used are for turbulent air flow in a 315 mm duct with 6 m/s (0.47 m^{3}/s). The kinematic viscosity used to calculate the Reynolds Number is 15 10^{6} m^{2}/s.
Replace the parameters with the appropriate values for your application.
The calculator below can be used to calculate the friction coefficient if velocity and kinematic viscosity of the fluid are known.
The friction coefficient can also be estimated with the Moody diagram (SI based):
Download and print Moody Diagram
Simplified Moody Diagram  SI
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Fluid Mechanics
The study of fluids  liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time. 
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Systems for ventilation and air handling  air change rates, ducts and pressure drops, charts and diagrams and more.
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