Sewers, culverts and drain pipes run by gravity. Their capacity when flowing full depends on diameter, slope and pipe material.
How to use the Pipe Flow Calculator
- Inner diameter D
- Pipe material (Manning n)
- Manning n
- Pipe length
- Elevation drop
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the inner diameter, choose the material, and the pipe length and fall.
Inputs explained
| Field | What to enter |
|---|---|
| Inner diameter D | Units: mm, cm, m, in, µm |
| Pipe material (Manning n) | Choose one: PVC / HDPE, Copper / smooth steel, Concrete, Cast iron … |
| Manning n | A number |
| Pipe length | Units: m, cm, mm, km, in, ft … |
| Elevation drop | Units: m, cm, mm, km, in, ft … |
What each result means
- Slope = [Elevation drop]÷[Pipe length]×100
- Full-flow discharge = [Full-flow velocity]×pi×[Inner diameter D]^2÷4
Example values
How the main result changes when Inner diameter D changes and everything else stays at the example values above:
| Inner diameter D (mm) | Full-flow discharge (L/s) |
|---|---|
| 150 | 10.7688 |
| 225 | 31.7501 |
| 300 | 68.3778 |
| 375 | 123.9771 |
| 450 | 201.6005 |
| 600 | 434.1717 |
Formula
S = drop/length, V = (1/n)·(D/4)^{2/3}·S^{1/2}, Q = V·πD²/4
Worked example
Inputs — Inner diameter D: 300 mm; Pipe material (Manning n): Concrete; Pipe length: 100 m; Elevation drop: 0.5 m.
Results — Slope = 0.5 %; Full-flow velocity = 0.96735 m/s; Full-flow discharge = 68.37776 L/s.
Key terms
- Gravity flow
- Flow driven by elevation difference.
Frequently asked questions
Is a gravity pipe pressurised?
No – it flows by slope with a free surface (or just full).
What is a minimum sewer slope?
Enough for about 0.6 m/s self-cleansing velocity.
Why does a pipe carry slightly more at 94% full?
The air gap reduces friction more than area.
Does length matter?
Only through the slope (drop/length).