Calculator Inputs
Enter top dimensions and storage depth. Then either supply bottom dimensions directly, or leave them blank and use side slopes.
Formula Used
Bottom length = Top length − 2 × (Length side slope × Depth)
Bottom width = Top width − 2 × (Width side slope × Depth)
Top area (At) = Top length × Top width
Bottom area (Ab) = Bottom length × Bottom width
Mid-depth area (Am) = [(Top length + Bottom length) ÷ 2] × [(Top width + Bottom width) ÷ 2]
Volume = Depth ÷ 6 × (Ab + 4Am + At)
This method is well suited to rectangular detention basins with straight, linearly changing side slopes between the pond bottom and top water surface.
How to Use This Calculator
- Select metric or imperial units.
- Enter the top length, top width, and water depth.
- Either enter bottom dimensions directly, or leave them blank.
- If bottom dimensions are blank, provide side slopes in H:1V format.
- Click the calculate button to show the storage result above the form.
- Review the summary table, graph, and export the results as CSV or PDF.
Example Data Table
| Case | Unit System | Top Length | Top Width | Bottom Length | Bottom Width | Depth | Volume |
|---|---|---|---|---|---|---|---|
| Example 1 | Metric | 40 m | 24 m | 28 m | 12 m | 3 m | 1,872 m³ |
| Example 2 | Imperial | 120 ft | 80 ft | 60 ft | 40 ft | 10 ft | 56,000 ft³ |
Frequently Asked Questions
1) What does this calculator estimate?
It estimates detention pond storage volume using top dimensions, bottom dimensions, and water depth. It also reports top area, bottom area, slope ratios, and converted output values.
2) Can I leave the bottom dimensions blank?
Yes. Leave bottom length and bottom width blank, then enter side slopes in H:1V format. The calculator will derive the bottom dimensions automatically.
3) What side slope format should I enter?
Enter horizontal distance per one vertical unit. For example, a 3H:1V side slope should be entered as 3. Separate length-side and width-side slopes can be used.
4) Which volume equation is used?
The page uses the prismoidal formula. It is a strong choice when top and bottom sections are rectangular and dimensions change linearly with depth.
5) Does the result include freeboard?
No. This result reflects the entered storage water depth only. Add freeboard separately if your project standards require extra embankment height above the design water level.
6) Which units are supported?
The calculator supports metric and imperial entry. It also converts the result to cubic meters, cubic feet, liters, gallons, and acre-feet for easier reporting.
7) Can I export the results?
Yes. After calculation, use the CSV button for spreadsheet-friendly output or the PDF button for a quick formatted report you can share or archive.
8) Is this enough for final design approval?
It is useful for planning, checking, and preliminary sizing. Final design should still be reviewed against grading, hydrology, outlet control, sediment storage, and local requirements.