BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Darcy friction factor f
- What it is
- The dimensionless Darcy (Moody) friction factor that sets how much pressure the pipe wall friction takes, found from the Reynolds number and the relative roughness of the pipe.
- Units and format
- Enter a number in dimensionless: dimensionless. The form accepts 0.001 dimensionless to 1 dimensionless. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.025 dimensionless as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop is directly proportional to the friction factor.
- Get it right
- Some references give the Fanning factor, which is one quarter of the Darcy factor; using it understates the loss four times.
What this means: Straight pipe length
- What it is
- The length of straight pipe the liquid flows along, measured along the pipe centreline.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 100000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop is directly proportional to length.
- Get it right
- Fittings and valves are not included; add their losses separately rather than guessing extra length.
What this means: Internal bore diameter
- What it is
- The inside diameter of the pipe the liquid flows through, used for the length-to-diameter ratio in the Darcy–Weisbach equation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.1 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- At a fixed velocity, the pressure drop is inversely proportional to bore.
- Get it right
- Use the internal bore, not the nominal size or outside diameter.
What this means: Mean flow velocity
- What it is
- The average speed of the liquid across the pipe cross-section: volumetric flow divided by bore area.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0 m/s to 100 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop rises with the square of velocity: doubling it quadruples the loss.
- Get it right
- The Pipe Flow Velocity tool converts flow and bore into this velocity; do not enter a pump outlet or peak velocity.
What this means: Fluid density
- What it is
- The mass per unit volume of the liquid or gas in the pipe at its operating temperature.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.01 kg/m³ to 25000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Pressure drop is directly proportional to density; head loss in metres of fluid does not depend on it.
- Get it right
- Take density at the operating temperature from a fluid data sheet; oils and hot water differ from cold 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.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Pipe flow, pressure drop and pump selection: getting fluid where it needs to go efficiently
In Fluids, HVAC and Building Services
What it covers: How head, velocity, friction and fittings set pressure drop, how system and pump curves fix the operating point, why efficiency and NPSH matter, and how pipe size affects energy cost.
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