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Hagen-Poiseuille equation

Fluid dynamics index

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: m --- METRIC --- pm nm microns (µm) mm cm km -- IMPERIAL -- mil 1/16 inch inches feet yards miles - SCIENTIFIC - Planck Bohrs Angstrom light-seconds light-years au parsecs --- OTHER --- points cubits fathoms rods chains football fields furlongs Roman miles nautical miles leagues Pipe length: m --- METRIC --- pm nm microns (µm) mm cm km -- IMPERIAL -- mil 1/16 inch inches feet yards miles - SCIENTIFIC - Planck Bohrs Angstrom light-seconds light-years au parsecs --- OTHER --- points cubits fathoms rods chains football fields furlongs Roman miles nautical miles leagues Velocity: m/s --- METRIC --- cm/s km/s -- IMPERIAL -- inches/second feet/second feet/minute - RECIPROCAL - min/mile min/km min/5 km min/10 km --- OTHER --- km/h mph miles/second knots furl's/f'night Mach c (light speed) Discharge rate: m³/s mL/min liter/min liter/s m³/min cu ft/s cu ft/min gal(US)/min gal(UK)/min Dynamic viscosity: Pa⋅s cP Pressure drop: Pa nPa µPa mPa kPa MPa GPa atm kgf/cm²   (at) mbar bar mmHg (Torr) in Hg ft H2O m H2O psi Add

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.