Calculator · Fluid Mechanics

Darcy–Weisbach Pressure Drop Calculator

The major loss of a pipe run: the head the fluid loses to wall friction over a length L, and the pressure drop that head represents. The loss is a nonnegative magnitude — in a system equation it always opposes the flow.

Inputs

Darcy friction factor f_D[-]

From the Darcy Friction Factor calculator; a Fanning factor must be converted first.

Pipe length L[m]

Hydraulic diameter D_h[m]

Mean speed V[m/s]

Fluid density ρ[kg/m³]

Results

Major head loss h_f = f_D (L/D_h) V²/(2g)

4.0789 m

A nonnegative magnitude; the system equation supplies the sign.

Darcy friction factor f_D	0.02
Pipe length L	100 m
Hydraulic diameter D_h	0.1 m
Mean speed V	2 m/s
Fluid density ρ	1000 kg/m³
Major head loss h_f	4.0789 m
Pressure drop Δp = ρ g h_f	40 kPa

Assumptions

· Fully developed, steady, incompressible flow.
· Darcy friction factor and Darcy–Weisbach form.

Explore in the Lab

Predict this loss from the pipe itself in the Pipe Flow Lab

The Lab derives the friction factor from the Reynolds number and the roughness instead of taking it as an input.

Technical reference

h_f = f_D (L / D_h) V² / (2g)

Δp = ρ g h_f

V = Q / A

h_f: major head loss along the pipe[m]
f_D: Darcy friction factor[-]
L: pipe length[m]
D_h: hydraulic diameter[m]
V: mean speed[m/s]
ρ: density of the fluid[kg/m³]

The head loss is a magnitude. Directional system equations apply it against the flow, so reversing the flow can never turn friction into an energy source.

Δp = ρ g h_f is the pressure drop only when the pipe has a constant area and no elevation change; otherwise use the full energy equation.

Reference case: f_D = 0.02, L = 100 m, D = 0.1 m, V = 2 m/s and ρ = 1000 kg/m³ give h_f ≈ 4.0789 m and Δp ≈ 40.00 kPa.