Pipe pressure drop: Darcy–Weisbach explained

Learn how to estimate pressure loss in a straight pipe with the Darcy–Weisbach equation, then run the transparent bilingual calculator with your own fluid, diameter, flow, material, and length.

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01 / The pressure-drop equation, in context

The pressure-drop equation, in context

For a straight, fully developed pipe run, the Darcy–Weisbach equation expresses friction loss as a function of the pipe geometry, fluid properties, velocity, and friction factor.

Darcy–Weisbach pressure drop

ΔP = f × (L / D) × (ρv² / 2)

Mean velocity from flow rate

v = 4Q / (πD²)

What each variable means

ΔP

Pressure drop

The friction loss across the selected straight pipe length, in Pa, kPa, or psi.

Darcy friction factor

A dimensionless factor based on Reynolds number and relative roughness.

L / D

Length and diameter

The pipe length and internal diameter, using the same unit system.

ρ, v

Density and velocity

Fluid density in kg/m³ and mean velocity in m/s determine the velocity pressure.

02 / A practical calculation sequence

A practical calculation sequence

01
Choose the fluid, internal diameter, pipe material, flow rate, and straight-run length.
02
Convert the inputs to consistent SI units and calculate mean velocity and Reynolds number.
03
Estimate the Darcy friction factor from the flow regime and relative roughness.
04
Review the pressure loss, gradient, assumptions, and preliminary-design disclaimer before using the result.

03 / Pipe pressure-drop questions