KEVOS® MAKEOS · Hydraulics
Darcy Pipe Pressure Drop
How much pressure is lost to friction along this straight pipe?
Calculate the friction pressure drop and head loss for fully developed flow in a straight circular pipe using the Darcy–Weisbach equation.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Darcy friction factor f | dimensionless | Darcy (Moody) friction factor for the Reynolds number and relative roughness, from a Moody chart or a friction-factor equation. A Fanning factor is one quarter of the Darcy factor. |
| Straight pipe length | m | Length of straight pipe. |
| Internal bore diameter | mm | Internal bore. |
| Mean flow velocity | m/s | Mean velocity; the Pipe Flow Velocity tool calculates it from flow and bore. |
| Fluid density | kg/m³ | Fluid density at the operating temperature, for example about 1,000 kg/m³ for water. |
Method and formulas
- Δp (Pa) = f × (L / D) × ρ × v² / 2, with D in metres.
- Head loss (m) = Δp / (ρ × g), with g = 9.80665 m/s².
- Example: f = 0.025, 20 m of 25 mm bore, 2 m/s and 1,000 kg/m³ give 0.025 × 800 × 2,000 = 40,000 Pa = 40 kPa, a head loss of 4.078865 m.
Limitations
- Straight pipe only: fittings, valves, entries, exits, elevation change and the pump itself are excluded.
- The answer is only as good as the friction factor; it must match the Reynolds number and roughness, and for laminar flow f = 64 / Re.
- Single-phase flow of approximately constant density; not for compressible gas with a large pressure drop.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate line losses for pump or circuit sizing
- Compare pipe sizes
- Check a measured pressure drop
Who it is for
- Design engineers
- Maintenance engineers
What you get
- Friction pressure drop (kPa and bar)
- Friction head loss (m of fluid)
Interpreting the result
- Pressure drop rises with velocity squared, so a modest increase in bore can reduce losses greatly.
Before relying on the result
- Confirm the factor is a Darcy factor and matches the Reynolds number from the Liquid Pipe Velocity and Reynolds Number tool.
- Add minor losses for fittings and valves before sizing a pump.
References
- US Department of Energy, DOE-HDBK-1012/3-92, Fluid Flow — https://www.energy.gov/sites/default/files/2026-04/DOE-HDBK-1012-92_VOL3.pdf
- Moody L. F., “Friction factors for pipe flow”, Transactions of the ASME 66 (1944), 671–684.
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