How do you estimate stormwater storage volume?

For a simplified constant-flow event, storage can be estimated from the difference between inflow and controlled outflow multiplied by event duration. If outflow is greater than inflow, the modeled incremental storage requirement is zero.

Does stormwater storage equal total runoff volume?

Not necessarily. Required storage depends on inflow timing, outlet control, initial storage, allowable discharge and the design event.

Can this calculator size a detention tank for approval?

No. It is a screening calculation and does not replace the hydraulic routing, design storm and regulatory method required for a project.

Runoff and detention planning

Stormwater Storage Volume Calculator — Event Storage Requirement

Estimate the volume that must be temporarily stored when stormwater inflow exceeds controlled outflow. The calculator supports a simple flow-duration model and a separate rainfall-runoff volume check.

Choose a storage model

Calculation method

Event duration (hours)

Average inflowUse litres/second.

Controlled outflowUse litres/second.

Existing available storageEnter m³ already available before the event.

Catchment areaSquare metres.

Rainfall depthMillimetres for the selected event.

Runoff coefficient

Collection efficiency

This is a screening model. Real detention design normally requires time-varying inflow, outlet control, routing, starting conditions and the project-specific design event.

Enter the event assumptions.

Stormwater storage result

Estimated required storage

—

—Event volume

—Controlled release

—Net accumulation

—With initial storage

—

Storage volume methodology

Flow-duration method

When inflow and outflow are treated as constant, the temporary accumulation is the excess inflow multiplied by event duration.

Inflow volume = Qin × time
Outflow volume = Qout × time
Incremental storage = max(0, inflow volume − outflow volume)
Required new storage = incremental storage − existing available storage, not less than zero

Rainfall-runoff check

Rainfall depth can also be converted into a runoff volume using catchment area and a runoff coefficient. A collection-efficiency factor can be used when the theoretical runoff should be reduced for a planning estimate.

Runoff volume = area × rainfall depth × coefficient × efficiency

Worked example

At 20 L/s inflow, 5 L/s outflow and a 2-hour event:

Net rate = 20 − 5 = 15 L/s
Storage = 15 × 7,200 seconds = 108,000 L = 108 m³

That value represents constant-rate accumulation under the simplified assumptions, not a regulatory detention volume.

Why real stormwater storage can differ

Rainfall intensity is rarely constant. Inlet flow can rise sharply, outlet flow may depend on water level, and upstream areas can contribute at different times. Existing storage, infiltration and emergency overflow also affect the actual design balance.

Use the result as an early-stage check before moving to the project's required hydraulic model.

Frequently Asked Questions

Can the result be zero?

Yes. If controlled outflow fully matches or exceeds the modeled inflow, no additional storage is required under the simplified model.

Does this calculate tank dimensions?

No. It calculates the storage volume target.

Can rainfall runoff be estimated directly?

Yes. Switch to the rainfall runoff model.

Is this suitable for flood-risk certification?

No. Project-specific hydraulic design and applicable requirements must be used.