How do you calculate catch basin volume?

For a rectangular basin, multiply internal length by internal width by internal depth. For a cylindrical basin, multiply pi by the radius squared and then by internal depth.

What is usable catch basin volume?

Usable volume is the gross internal volume reduced by any freeboard or other volume that should not be counted as available storage.

Does this calculate hydraulic inlet capacity?

No. Volume is only one part of catch-basin performance; inlet, outlet, grate, sediment and overflow characteristics may control actual operation.

Stormwater geometry

Catch Basin Capacity Calculator — Internal Volume & Working Storage

Convert internal catch-basin dimensions into gross volume, litres, cubic metres and an adjusted working volume after a user-defined freeboard allowance.

Basin dimensions

Basin shape

Dimension unit

Internal length

Internal width

Internal diameter

Internal depth

Excluded volume / freeboard (%)

Use clear internal dimensions. This calculator does not deduct sediment volume, inlet structures, outlet structures or irregular internal geometry unless you model those separately.

Enter the internal basin dimensions.

Catch basin volume

Working storage volume

—

—Gross volume

—Working volume

—Working litres

—Excluded volume

—

Catch basin volume formulas

For a rectangular basin, capacity is the product of its three internal dimensions. For a cylindrical basin, capacity is the circular base area multiplied by internal depth.

Rectangle = length × width × depth
Cylinder = π × (diameter ÷ 2)² × depth
Working volume = gross volume × (1 − excluded percentage)

Why internal dimensions matter

External chamber dimensions include wall thickness and therefore do not represent storage volume. Use internal clear dimensions for a geometric capacity estimate.

Capacity is not flow performance

A larger chamber volume does not automatically mean a drainage system can accept or discharge a larger flow. Inlet area, outlet size, grate configuration, sediment and hydraulic controls must be assessed separately.

Worked example

A 1.2 m × 1.0 m × 1.5 m rectangular chamber has 10% of its gross volume excluded from working storage.

Gross = 1.2 × 1.0 × 1.5 = 1.80 m³
Excluded = 1.80 × 10% = 0.18 m³
Working volume = 1.80 − 0.18 = 1.62 m³ = 1,620 L

Where the estimate can differ from a real chamber

Actual storage may be reduced by benching, pipe intrusions, internal baffles, pumps, sediment accumulation and other features. An irregular chamber also needs a more detailed geometric model than the basic shapes available here.

For formal stormwater design or flood-control decisions, use project-specific drawings, survey data and the required hydraulic methodology.

Frequently Asked Questions

Can I enter millimetres?

Yes. The calculator converts millimetres to metres.

What does freeboard mean here?

It is a user-defined percentage of gross volume that you choose not to count as available working storage.

Does this calculate the correct catch-basin size?

No. It evaluates the dimensions you enter.

Can a cylindrical basin be calculated?

Yes. Select Cylindrical and enter diameter and depth.