Retention Basin Calculator for Stormwater Storage

Enter Design Inputs

Catchment area

Area unit

Runoff coefficient (C)

Rainfall depth

Rainfall depth unit

Rainfall intensity

Intensity unit

Safety factor

Outlet release rate

Outlet unit

Drawdown period (hours)

Total basin depth (m)

Freeboard (m)

Side slope (H:1V)

Length to width ratio

Sediment storage allowance (m³)

Formula Used

1) Raw runoff volume
Vraw = A × P × C

2) Safety-adjusted runoff volume
Vdesign = Vraw × Safety Factor

3) Outlet release during drawdown
Vrelease = Qout × t

4) Required storage volume
Vrequired = max(Vdesign − Vrelease, 0) + Vsediment

5) Basin volume for a rectangular frustum
V = d / 3 × (Ab + At + √(Ab × At))

Where:
A = catchment area, P = rainfall depth, C = runoff coefficient, Qout = outlet release rate, t = drawdown time, d = usable water depth, Ab = bottom area, and At = top area.

How to Use This Calculator

Enter the total catchment area and select the correct area unit.
Input a runoff coefficient that reflects pavement, roofs, soil, or mixed site conditions.
Enter rainfall depth for the selected design storm.
Optionally enter rainfall intensity to estimate peak runoff rate.
Provide outlet release rate and drawdown period to account for controlled discharge.
Enter total basin depth, freeboard, side slope, and length-to-width ratio.
Add any sediment storage allowance you want reserved below working storage.
Press calculate to view storage volume, basin dimensions, drain time, and the stage-storage graph.
Use the CSV or PDF buttons to export the calculated summary.

Example Data Table

Catchment Area	Runoff Coefficient	Rainfall Depth	Intensity	Outlet Rate	Drawdown	Total Depth	Freeboard	Side Slope	L:W Ratio	Sediment Storage	Approx. Required Storage
2.5 ha	0.70	75 mm	45 mm/hr	10 L/s	24 hr	2.4 m	0.3 m	3:1	2.0	80 m³	≈ 725.38 m³

Example values are included for quick testing. Actual design should follow local hydrology, overflow routing, and authority requirements.

Frequently Asked Questions

1) What does a retention basin calculator estimate?

It estimates stormwater runoff volume, required storage, outlet release effect, approximate drain time, and basin dimensions based on depth, side slopes, and design assumptions.

2) What is the runoff coefficient?

The runoff coefficient represents how much rainfall becomes runoff. Hard surfaces usually have higher values, while lawns and permeable ground usually have lower values.

3) Why is freeboard important?

Freeboard is the vertical safety margin above the design water level. It helps prevent overtopping during uncertainty, wave action, clogging, and unusual storm behavior.

4) What does drawdown period mean?

Drawdown period is the time allowed for stored water to leave the basin through the outlet system. It affects how much storage must remain available during and after the storm.

5) Why add sediment storage allowance?

Sediment storage reserves extra space below the active water storage zone. This helps maintain performance over time before maintenance removes accumulated solids.

6) Does this calculator replace detailed drainage design?

No. It is a planning and preliminary sizing tool. Final design should include local rainfall standards, routing checks, soil conditions, outlet control details, and code compliance.

7) What shape does this calculator assume?

It assumes a rectangular basin footprint with side slopes, creating a frustum-like storage shape. That lets the tool estimate top and bottom dimensions consistently.

8) Can I use acres, inches, or cfs?

Yes. The calculator accepts multiple common units for area, rainfall depth, rainfall intensity, and outlet discharge, then converts them into base units internally.