Darcy–Weisbach Pressure Loss Calculator

Calculate pressure loss across pipes and fittings. Analyze turbulent or laminar flow with practical inputs. See exports, graphs, formulas, examples, and usage guidance instantly.

Calculator Input Form

Input mode

Flow rate

Velocity

Pipe length

Pipe diameter

Pipe material preset

Absolute roughness

Fluid density

Dynamic viscosity

Total minor loss coefficient K

Gravity

m/s2

Example Data Table

Case	Length	Diameter	Flow	Roughness	Total K	Approximate Total Loss
Water, commercial steel	100 m	80 mm	36 m3/h	0.045 mm	2.5	≈ 53.9 kPa
Water, PVC	50 m	50 mm	12 m3/h	0.0015 mm	1.2	≈ 27.3 kPa
Light oil, cast iron	120 m	100 mm	25 m3/h	0.26 mm	3.0	≈ 20.8 kPa

Formula Used

The calculator uses the Darcy–Weisbach relationship for major pressure loss in straight pipe sections:

ΔP_major = f × (L / D) × (ρV² / 2)

Minor losses from fittings, bends, valves, and entrances are added with:

ΔP_minor = K × (ρV² / 2)

The total pressure loss is:

ΔP_total = ΔP_major + ΔP_minor

Velocity is obtained from flow rate and internal pipe area. Reynolds number is calculated as Re = ρVD / μ.

For laminar flow, friction factor is f = 64 / Re. For turbulent flow, the calculator solves the Colebrook–White equation iteratively. Transitional flow is smoothly interpolated between laminar and turbulent behavior for a practical estimate.

How to Use This Calculator

Select whether you want to enter flow rate or fluid velocity.
Enter pipe length and internal diameter with matching units.
Choose a material preset or enter custom absolute roughness.
Provide fluid density and dynamic viscosity values.
Enter the total minor loss coefficient for fittings and valves.
Press the calculation button to see the result above the form.
Review pressure loss, head loss, Reynolds number, and friction factor.
Use the CSV or PDF buttons to save the report.

Frequently Asked Questions

1. What does the calculator estimate?

It estimates major pipe friction loss, minor fitting loss, total pressure loss, and equivalent head loss for incompressible flow inside a circular pipe.

2. When should I use Darcy–Weisbach instead of Hazen–Williams?

Use Darcy–Weisbach when you need a physics-based method that handles different fluids, viscosities, and flow regimes more consistently than empirical water-only methods.

3. Why is Reynolds number important?

Reynolds number determines whether flow is laminar, transitional, or turbulent. That choice directly affects the friction factor and the final pressure drop estimate.

4. What is absolute roughness?

Absolute roughness represents the average wall texture height inside the pipe. Rougher materials usually create more turbulence and larger friction losses.

5. What does the total K value include?

Total K combines minor loss coefficients for elbows, tees, valves, entrances, exits, reducers, and other local disturbances placed along the line.

6. Can I enter velocity instead of flow rate?

Yes. The calculator can start from velocity, then internally converts it to flow rate using the selected pipe diameter.

7. Does the calculator work for gases?

It is best suited for incompressible conditions. Gas systems with significant density change need compressible-flow methods and added thermodynamic corrections.

8. Why are pressure loss and head loss both shown?

Pressure loss is useful for pumps and system pressure checks. Head loss is convenient when comparing energy loss per unit weight of fluid.