# Pipe Flow Calculator (Manning, Gravity Pipe)

URL: https://speedycalculator.com/calculators/physics/pipe-flow/
Last updated: 2026-10-30

> The pipe flow calculator finds the full-bore velocity and discharge of a gravity (non-pressurised) pipe from its diameter, material roughness and the drop over its length, using Manning's equation.

## Inputs

- Inner diameter D (mm, cm, m, in, µm)
- Pipe material (Manning n)
- Manning n
- Pipe length (m, cm, mm, km, in, ft, yd, mi)
- Elevation drop (m, cm, mm, km, in, ft, yd, mi)

## Results

- Slope
- Full-flow velocity
- Full-flow discharge

## Formula

```
S = drop/length,  V = (1/n)·(D/4)^{2/3}·S^{1/2},  Q = V·πD²/4
```

## Explanation

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

FieldWhat to enterInner diameter DUnits: mm, cm, m, in, µmPipe material (Manning n)Choose one: PVC / HDPE, Copper / smooth steel, Concrete, Cast iron …Manning nA numberPipe lengthUnits: m, cm, mm, km, in, ft …Elevation dropUnits: 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)15010.768822531.750130068.3778375123.9771450201.6005600434.1717

## 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.

## FAQ

**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).


## Sources

- [Fluid Mechanics (F. M. White) – reference text](https://www.mheducation.com/highered/product/fluid-mechanics-white-chul/M9781260258318.html)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)