darcy weisbach équation

Overview

The Darcy-Weisbach equation relates pressure loss due to friction along a pipe to the flow velocity, pipe diameter, and friction factor. It is the most widely used equation for pipe pressure drop calculations.

Key Formulas

Darcy-Weisbach Equation

$\Delta P = f \cdot \frac{L}{D} \cdot \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Head Loss Form

$h_f = f \cdot \frac{L}{D} \cdot \frac{v^2}{2g}$

Head loss in terms of fluid column height.

Darcy Friction Factor (Laminar)

$f = \frac{64}{Re}$

For laminar flow (Re < 2300).

Variables

Symbol	Description	Unit
$\Delta P$	Pressure drop	Pa
$f$	Darcy friction factor	—
$L$	Pipe length	m
$D$	Pipe inner diameter	m
$\rho$	Fluid density	kg/m³
$v$	Flow velocity	m/s
$h_f$	Head loss	m

Darcy-Weisbach Pressure Drop Calculator

Notes

• The Darcy friction factor is 4× the Fanning friction factor — be careful with references
• For turbulent flow, use the Colebrook equation or Moody chart for friction factor
• For laminar flow (Re < 2300), f = 64/Re exactly

References

• Original Source