Darcy-Weisbach Calculator – Head Loss

What this calculator does

Calculates pipe velocity, Reynolds number, Darcy friction factor, straight-pipe head loss and pressure drop. Turbulent friction factor is solved iteratively from Colebrook–White.

Inputs

Flow rate m³/h

Inside diameter mm

Pipe length m

Absolute roughness mm

New commercial steel is often near 0.045 mm; use project-specific data.

Density kg/m³

Dynamic viscosity cP

Pump efficiency %

Flow rate, diameter, density and viscosity must be positive; length and roughness may be zero; efficiency must be 0–100%, and ε/D must not exceed 0.1.

The flow lies in the transitional regime; friction factor is especially uncertain.

Results

Head loss

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Pressure drop

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Velocity

—

Reynolds number

—

Darcy friction factor

—

Estimated shaft power

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Regime: —

What Darcy–Weisbach calculates

The Darcy–Weisbach equation calculates the irreversible head or pressure loss caused by wall friction in a straight, constant-diameter pipe: hL = f(L/D)V²/(2g).

Why this method is preferred

It is dimensionally consistent and applies to liquids and gases when the appropriate density, viscosity and friction factor are used. Unlike an empirical water-only relation, viscosity enters explicitly through Reynolds number.

How friction factor is selected

Laminar flow uses f = 64/Re. Turbulent flow uses the Colebrook–White relation with Reynolds number and relative roughness ε/D. The displayed factor is the Darcy factor, not the Fanning factor.

How to use the inputs

Use actual internal diameter, developed straight-pipe length and absolute roughness. Fluid density and dynamic viscosity should correspond to the operating state. Pump efficiency converts hydraulic loss power into estimated shaft input power.

What is not included

The result covers straight-pipe wall friction only. Add valves, bends, tees, strainers, entrances, exits and equipment losses using loss coefficients, equivalent lengths or manufacturer pressure-drop data.

Darcy–Weisbach Calculator