Darcy–Weisbach Pressure Drop Calculator (SI Units)
Estimate frictional pressure loss and head loss in a straight pipe. Enter known values below and click Calculate.
What the Darcy–Weisbach equation calculates
The Darcy–Weisbach equation is one of the most reliable ways to estimate pressure loss caused by friction in internal flow. It applies to liquids and gases flowing through pipes and ducts, and it scales correctly across many pipe sizes and flow rates.
In pressure form: ΔP = f · (L/D) · (ρv²/2)
In head-loss form: hf = f · (L/D) · (v²/2g)
- ΔP = pressure drop (Pa)
- hf = head loss (m of fluid)
- f = Darcy friction factor (dimensionless)
- L = pipe length (m)
- D = internal pipe diameter (m)
- ρ = fluid density (kg/m³)
- v = average velocity (m/s)
How this calculator determines friction factor
1) Auto mode
In auto mode, the tool computes Reynolds number from density, viscosity, diameter, and velocity. Then:
- Laminar flow (Re < 2300): uses f = 64/Re
- Turbulent flow (Re ≥ 4000): uses the Swamee–Jain explicit approximation
- Transition range (2300–4000): blends laminar and turbulent estimates for a smooth estimate
This gives practical engineering estimates without solving the Colebrook equation iteratively.
2) Manual mode
If you already know the Darcy friction factor from measurements, design standards, or a Moody chart, select manual mode and enter f directly.
When to use this calculator
- Estimating pump pressure requirements for water distribution
- Sizing pipes for industrial process lines
- Comparing pressure losses between alternative diameters
- Checking whether a velocity target causes excessive energy loss
Quick example
Suppose water flows through a 50 m pipe with diameter 0.1 m at 2 m/s, with density 998 kg/m³, viscosity 0.001 Pa·s, and roughness 0.000045 m. In auto mode, the calculator estimates:
- Reynolds number and flow regime
- Darcy friction factor
- Head loss in meters of fluid
- Pressure drop in Pa and kPa
Try changing diameter while keeping flow velocity constant—you’ll see pressure drop shift dramatically because the equation contains L/D.
Important assumptions and limitations
- Flow is steady and fully developed
- Pipe is straight and circular
- Minor losses from fittings, valves, bends, and entrances are not included
- Fluid properties are treated as constant
For full system design, combine this friction loss with minor-loss coefficients (K values) and elevation changes.
Tips for reducing pressure loss
- Increase pipe diameter where possible
- Reduce unnecessary velocity spikes
- Use smoother pipe materials/coatings
- Minimize sharp bends and restrictive fittings
- Keep pipelines clean to limit roughness growth over time
Final note
The Darcy–Weisbach method is a foundational tool in fluid mechanics and piping design. Use this calculator for quick, transparent estimates, then validate with project-specific standards, fluid-property data, and safety factors for final engineering decisions.