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Pipe & Open Channel Flow Calculator

Manning Equation Calculator

Calculate flow capacity and velocity for pipes and open channels, and compare capacity with Rational method peak runoff.

by Shahid Pervaiz: pervaiz.shahid@gmail.com

ManningFlow
Rational method → Manning's equation → capacity check
Not saved to a file yet

Unit system

Choose metric or US customary — all fields and formulas below update to match

Q = C · I · A / 360
V = (1/n) · R^(2/3) · S^(1/2)
Q_cap = V · A

Project

Shown on the printout and report

1 · Catchment — Rational method

Peak runoff discharge feeding the pipe or channel

Selecting a terrain type overwrites the C field above.

2 · Conveyance geometry

Pipe or open channel — choose a cross-section

Longitudinal fall
Roughness coefficient — see reference table
VELOCITY
0.000m/s
V = (1/n)·R^(2/3)·S^(1/2)
PIPE / CHANNEL CAPACITY
0.000m³/s
Q = V · A

Hydraulic properties

Circular pipe

Flow area
0.00 m²
Wetted perimeter
0.00 m
Hydraulic radius
0.00 m
Water depth
0.00 m
Top width
0.00 m
Full-flow capacity
0.00 m³/s

Capacity vs. peak runoff

Rational-method design-storm check

Peak runoff, Q_peak
0.000 m³/s
Conveyance capacity
0.000 m³/s
Capacity / Demand—
Enter values to check capacity.
Method & practical use

Understand the result.

Calculate flow capacity and velocity for pipes and open channels, and compare capacity with Rational method peak runoff.

How to use this tool

  1. Select the geometry or design case and confirm the displayed units.
  2. Replace sample inputs with the project values, then calculate or review the live result.
  3. Check the method, assumptions and output; save or print the calculation with its input record.

Units & method

Metric: m, m², m³/s; rainfall mm/h and area ha. US: ft, ft², cfs; rainfall in/h and area acres.

Q = (k/n) A R^(2/3) √S; R = A/P. k is 1 in SI and 1.49 in US units. Enter slope as a percentage; the calculation uses its decimal. Uniform, gravity-driven flow is assumed. The US Rational method uses the conventional Q ≈ CiA approximation.

Worked examples

Illustrative inputs for checking the method; these are not project records.

Full circular pipe, METRIC

D = 1 m, n = 0.013 and slope 1%. A = 0.7854, R = 0.2500; Q = 2.398 m³/s and velocity = 3.053 m/s.

Full circular pipe, US

D = 3 ft, n = 0.013 and slope 0.5%. A = 7.0686, R = 0.7500; Q = 47.290 cfs and velocity = 6.690 ft/s.

Frequently asked questions

What does this tool calculate or track?

Calculate flow capacity and velocity for pipes and open channels, and compare capacity with Rational method peak runoff.

Which units and assumptions should I use?

Metric: m, m², m³/s; rainfall mm/h and area ha. US: ft, ft², cfs; rainfall in/h and area acres. Q = (k/n) A R^(2/3) √S; R = A/P. k is 1 in SI and 1.49 in US units. Enter slope as a percentage; the calculation uses its decimal. Uniform, gravity-driven flow is assumed. The US Rational method uses the conventional Q ≈ CiA approximation.

Can I save or share the result?

Use the save, export and print controls shown in this workspace. Where CSV export is available, the exported data can be opened in a spreadsheet. Browser print can save a PDF.

Continue your workflow

How to use the Pipe and Open Channel Flow Calculator (Manning's Equation)

Everything runs in your browser — nothing is uploaded. If you are new to the tool, follow these steps in order.

1

Enter your inputs and read the results

Choose Metric (SI) or US Customary (FPS) units at the top, then work down the page. Results update as you type, so there is no Compute button to press.

  • Project — project or drain name and who prepared it.
  • 1 · Catchment — Rational method — runoff coefficient C (or quick-set it from the terrain and cover), catchment area and rainfall intensity give the peak runoff Q = CiA.
  • 2 · Conveyance geometry — choose a circular pipe, rectangular, triangular or trapezoidal channel and enter its size, bed slope and Manning's n. Set the water depth, or use 25% 50% 75% Full.
  • Hydraulic properties and Capacity vs. peak runoff — flow area, wetted perimeter, hydraulic radius, velocity and capacity, checked against the peak runoff.
2

Save your project and open it later

💾 Save Project (.json) saves every input on the page in one .json project file. 📁 Load Project opens that file again, on any computer.

The note beside the Save button tells you whether you have unsaved changes or when you last saved. If you try to close or leave the page with unsaved changes, the browser asks you to confirm first.

Use the project file for backups and for sharing with colleagues. Don't edit it by hand.

This tool does not autosave. Save your project before closing the page.

3

Print, PDF and report

  • 🖨 Print / Save as PDF prints the inputs, results and cross-section. To get a PDF, choose Save as PDF as the printer in the print dialog.
  • 📄 Generate Report opens the full design report in a new tab, with its own Print / Save as PDF button. If nothing opens, allow pop-ups for this site.
4

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