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Hagen-Poiseuille equation

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Hagen-Poiseuille equation diagram
This equation, used in hydraulics, fluid dynamics and civil engineering, states that ΔP=32μLV/d². This is a special case of the Darcy-Weisbach formula, when solved for incompressible fluids in laminar flow through circular pipes (where the friction factor can be calculated from conditions).
Pipe diameter:
Pipe length:
Discharge rate:
Dynamic viscosity:

Pressure drop:
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Note that you only need to enter either the velocity or the discharge rate (wichever you know). Also, be sure only to use this calculation for appropriate cases. For other conditions, other suitable calcs are available.
This equation assumes that all fluid flow is continually along the pipe, with the center moving fastest and the outside slowly (because of drag from the walls). Thus turbulent effects (appearing at ~Re=2000) are not taken into account.