Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Pipe pressure drop calculator
Estimate friction pressure drop and head loss in a straight, full liquid pipe using Darcy–Weisbach. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Flow rate (m³/h). Flow through the pipe, in m³/h. The value must be greater than zero.
- Internal diameter (mm). Actual internal diameter in mm, not the nominal size. The value must be greater than zero.
- Straight pipe length (m). Straight pipe length in metres. Add fittings separately as equivalent length.
- Absolute pipe roughness (mm). Absolute roughness in mm, for example about 0.045 mm for new commercial steel pipe.
- Liquid density (kg/m³). About 998 kg/m³ for water at 20 °C. The value must be greater than zero.
- Dynamic viscosity (mPa·s). Dynamic viscosity in mPa·s (the same as cP); about 1 for water at 20 °C. The value must be greater than zero.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
The Darcy–Weisbach equation links friction loss to pipe length, diameter, velocity and a friction factor. The factor depends on the Reynolds number and the pipe roughness. In laminar flow it is 64 divided by the Reynolds number; in turbulent flow the Swamee–Jain formula gives a close explicit estimate of the Colebrook–White equation. The result covers straight pipe only.
v = Q / A. Re = ρ × v × D / μ. Laminar (Re < 2000): f = 64 / Re. Turbulent: f = 0.25 / [log10(ε/(3.7 D) + 5.74 / Re^0.9)]² (Swamee–Jain). ΔP = f × (L / D) × ρ × v² / 2. Head loss = ΔP / (ρ × g).
This covers straight pipe only. Add valves, bends and fittings as equivalent length or with loss coefficients. It assumes an incompressible liquid in a full pipe, so it is not for gases, steam or two-phase flow. Between Re 2000 and 4000 flow is transitional and the result is indicative. Use the actual internal diameter, the viscosity at the operating temperature and a realistic roughness for aged pipe.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 0.1585 bar friction pressure drop. Straight-pipe friction only. Add fittings, elevation change and control valve drop to estimate the total system pressure requirement.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Using nominal instead of internal diameter; entering viscosity in the wrong unit (mPa·s equals cP, not Pa·s); using water viscosity for a cold or viscous liquid; leaving out fittings and valves; using the result for gas or steam. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The result is the friction pressure drop in the straight pipe, with the head loss and flow regime. Add fittings, elevation and valves to find the total pressure the pump must supply.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. This covers straight pipe only. Add valves, bends and fittings as equivalent length or with loss coefficients. It assumes an incompressible liquid in a full pipe, so it is not for gases, steam or two-phase flow. Between Re 2000 and 4000 flow is transitional and the result is indicative. Use the actual internal diameter, the viscosity at the operating temperature and a realistic roughness for aged pipe. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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