Formula used
Manning: Q = (1/n)AR2/3S1/2.
Hydraulic radius: R = A/P.
Velocity: V = Q/A.
Darcy-Weisbach: hf = f(L/D)(V²/2g).
Reynolds: Re = VD/ν.
Froude: Fr = V/√(gA/T).
Manning suits open channels and partly filled pipes. Hazen-Williams estimates turbulent water flow in full pipes. Darcy-Weisbach handles fluid properties and fitting losses accurately.
Manning and Hazen capacity uses the selected hydraulic slope. Their fitting losses appear separately in the results. Darcy flow directly includes every entered minor loss.
How to use
- Select the hydraulic equation and required calculation mode.
- Choose the section shape and enter its dimensions.
- Enter length, elevations, slope, material, and fluid data.
- Add fittings and define practical design limits.
- Submit the form and review warnings carefully.
- Export, print, copy, or save the calculation.
Example data
| Input | Example | Purpose |
|---|---|---|
| Pipe diameter | 450 mm | Defines circular geometry. |
| Depth ratio | 0.70 | Models partial flow. |
| Pipe length | 100 m | Sets travel and losses. |
| Slope | 1.0% | Provides gravity gradient. |
| Material | Concrete | Loads roughness values. |
| Fluid | Water at 20°C | Sets viscosity and density. |
Typical roughness guidance
| Material | Manning n | Hazen C | Roughness, mm |
|---|---|---|---|
| PVC | 0.009 | 150 | 0.0015 |
| HDPE | 0.01 | 150 | 0.007 |
| Concrete | 0.013 | 130 | 0.3 |
| Vitrified clay | 0.013 | 140 | 0.06 |
| Cast iron | 0.015 | 100 | 0.26 |
| Ductile iron | 0.012 | 120 | 0.26 |
| Commercial steel | 0.012 | 120 | 0.045 |
| Corrugated metal | 0.024 | 80 | 3 |
| Fibreglass | 0.01 | 150 | 0.005 |
| Brick | 0.015 | 100 | 3 |
| Existing aged pipe | 0.02 | 85 | 1.5 |
| Custom | 0.013 | 130 | 0.1 |
Published coefficients vary with age, joints, deposits, and workmanship. Confirm project values before approving final designs. Local standards should control every engineering decision.
Frequently asked questions
Which equation should I choose?
Use Manning for open or partly filled gravity conduits. Hazen-Williams is common for full water pipes. Darcy-Weisbach provides broader fluid and loss modelling.
Can this calculator size a pipe?
Yes, select the required diameter mode. Circular sections use an iterative diameter search. Always select a practical commercial size afterward.
Does it support partially full pipes?
Yes, circular segment geometry calculates area and wetted perimeter. Enter depth directly or use a ratio. The result includes percentage full.
How is slope entered?
Slope accepts decimal, percentage, ratio, or unit fall. Elevations can also define the gradient. Every format converts internally to decimal slope.
Are fitting losses included?
Yes, enter entrance, exit, bends, valves, and screens. Each coefficient contributes to total minor losses. Darcy-Weisbach uses these losses directly.
What is self-cleansing velocity?
It is a selected minimum velocity for limiting deposits. Required values depend on solids and standards. Adjust the warning threshold for your project.
Why can normal depth fail?
The target flow may exceed section capacity. Invalid slopes or dimensions can also prevent solutions. Review warnings and increase the search depth.
Does the drain time vary with head?
This estimate assumes a steady calculated discharge. Real tank head usually falls during drainage. Detailed transient modelling may produce longer times.
What does Froude number show?
Froude number compares inertia with gravity effects. Values below one indicate subcritical flow. Values above one indicate supercritical flow.
What does Reynolds number show?
Reynolds number compares inertial and viscous effects. It classifies laminar, transitional, or turbulent conditions. Pipe roughness matters most during turbulence.
Can results replace an engineer?
No, results are preliminary hydraulic estimates. Real systems require surveys, standards, and safety reviews. Qualified professionals should verify final designs.
Engineering disclaimer
This calculator provides educational and preliminary design estimates. It cannot represent every field condition or regulation. Verify all outputs with qualified engineering review.