Plan drainage using pipe size, slope, and materials. Review capacity, velocity, and partial flow performance. Save tables, exports, and graphs for accurate field decisions.
This table shows sample capacities using the current slope, roughness, and depth settings.
| Pipe Size (mm) | Capacity at Selected Depth (L/s) | Full-Flow Capacity (L/s) | Velocity at Selected Depth (m/s) |
|---|---|---|---|
| 100.00 | 7.293 | 7.461 | 1.083 |
| 150.00 | 21.502 | 21.998 | 1.419 |
| 200.00 | 46.308 | 47.376 | 1.719 |
| 250.00 | 83.962 | 85.898 | 1.994 |
| 300.00 | 136.531 | 139.679 | 2.252 |
| 375.00 | 247.548 | 253.255 | 2.613 |
This calculator uses Manning’s equation for gravity flow in circular pipes.
Flow rate: Q = (1 / n) × A × R^(2/3) × S^(1/2)
Velocity: V = Q / A
Hydraulic radius: R = A / P
Full pipe area: A = πD² / 4
Full pipe hydraulic radius: R = D / 4
For partial flow, the calculator uses exact circular segment geometry. It first calculates wetted area and wetted perimeter from the selected flow depth. Then it applies Manning’s equation using that hydraulic radius. This helps estimate how slope, roughness, diameter, and fill depth affect drainage performance.
It estimates drainage capacity, velocity, stored volume, and recommended pipe size using diameter, slope, roughness, fill depth, and pipe count.
It uses Manning’s equation with circular pipe geometry. Partial flow conditions are handled with wetted area and wetted perimeter calculations.
Slope drives gravity flow. A steeper slope usually increases velocity and capacity, while a flatter slope reduces drainage performance.
Manning n represents internal resistance. Smoother materials usually have lower values, which increases estimated flow capacity.
Yes. Enter the flow depth as a percent full. The calculator then uses partial-flow geometry instead of assuming the pipe runs full.
If you enter a design flow, the calculator checks common standard sizes and returns the first size that can handle that flow.
Stored volume helps estimate how much water sits inside the pipe run at the selected flow depth and length.
Yes. Use the CSV button for spreadsheet work and the PDF button for a printable project record.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.