A STORM score is calculated by modelling the runoff generated by a development, assessing the treatment measures connected to that runoff and comparing the resulting pollutant reductions with best-practice stormwater-quality objectives.
The calculation begins with the areas of the proposed development. Roofs, driveways, paths, paving and other impervious surfaces are entered as runoff-generating catchments.
Each catchment is then connected to a proposed treatment measure, such as a rainwater tank, raingarden or permeable surface, or identified as untreated.
The assessment tool estimates how effectively the combined treatment strategy reduces stormwater pollutants relative to the applicable best-practice objectives. The resulting percentage is presented as the project’s STORM or stormwater treatment score.
A score of 100% is generally understood to mean that the modelled design meets the relevant best-practice treatment benchmark. It does not mean that all runoff is removed, that no pollutants remain or that the project has satisfied every drainage and planning requirement.
The STORM score is a relative measure of stormwater treatment performance.
It compares the treatment achieved by the proposed development with the reference level established by Victoria’s best-practice stormwater-quality objectives.
Those objectives have traditionally included reductions in the typical urban annual loads of:
The score is therefore not a simple percentage of rainwater retained on site. It reflects how the modelled combination of catchments and treatment measures performs against the relevant water-quality benchmark.
The exact calculation is performed by the assessment tool. A project team would not normally calculate the final composite score manually.
For an overview of the broader assessment process, visit the Certified Energy STORM Knowledge Hub.
Stormwater treatment assessments in Victoria have historically been tested against established best-practice pollutant-reduction objectives.
These commonly include:
The calculator assesses how the proposed treatment strategy performs relative to these objectives.
A 100% result does not mean that 100% of each pollutant is removed. It indicates that the modelled performance reaches the benchmark represented by the tool.
The calculation first needs a clear assessment boundary.
For a typical development, this may be the complete title boundary or the area affected by the proposed buildings and works. The correct boundary depends on the project and the council requirement.
The total area is then divided into surface types, such as:
The areas entered into the model should reconcile with the total site or assessment area.
If impervious surfaces are omitted or underestimated, the score may not accurately represent the proposed development.
Impervious surfaces generate most of the runoff assessed by the tool.
These surfaces prevent rainfall from infiltrating naturally into the ground and instead direct water towards gutters, pits, drains or adjoining areas.
The assessor identifies the area of each catchment and determines where its runoff will go.
For example:
This allocation is central to the calculation. A treatment measure can only improve the score for the runoff that actually reaches it.
Once the catchments are defined, the assessor links them to the proposed stormwater treatment measures.
Common measures may include:
The assessment must reflect a credible physical relationship between the catchment and treatment measure.
For example, a roof can only be counted as draining to a tank where suitable gutters and downpipes can connect that roof to the tank. A driveway can only be assigned to a raingarden where the levels and drainage arrangement allow runoff to reach it.
Each treatment measure requires information describing its size, connection and intended operation.
The inputs vary according to the type of treatment.
Relevant inputs commonly include:
Relevant inputs may include:
The assessment may consider:
The score is only as reliable as the assumptions entered into the model.
Not every impervious surface will necessarily drain through a treatment measure.
Common untreated areas include:
These surfaces still need to be entered into the assessment.
A high-quality result should not depend on excluding inconvenient catchments from the model. The calculation should represent the actual proposed development, including both treated and untreated runoff.
The assessment tool uses its internal runoff and pollutant-treatment calculations to estimate the performance of each treatment measure.
It then combines the treated and untreated catchments to determine the overall project result.
In practical terms, the final score is influenced by:
The output is expressed as a percentage relative to the best-practice benchmark.
A score below 100% generally indicates that the modelled treatment strategy has not yet reached the reference best-practice objective.
This does not necessarily mean that the overall development has been rejected or that every element of the strategy is unsuitable.
It means that, based on the current inputs, further treatment or design refinement may be needed.
Potential responses can include:
The most appropriate change depends on which surfaces and treatment measures are limiting the result.
A 100% STORM score generally indicates that the modelled treatment strategy meets the benchmark represented by the assessment tool.
It does not mean:
The score relates specifically to the modelled stormwater treatment outcome.
Read What Does a 100% STORM Rating Mean? for a more detailed explanation.
Depending on the assessment output, a treatment strategy may perform above the minimum reference benchmark.
This can occur where the development includes substantial treatment relative to its impervious area.
However, a result above 100% should not be pursued at the expense of a practical and coordinated design.
The treatment measures still need to:
A mathematically strong result is not useful if the modelled strategy cannot be constructed or maintained.
Rainwater tanks can improve a STORM score by capturing roof runoff and creating storage capacity through ongoing reuse.
Three inputs work together:
A larger tank does not automatically produce a proportionally higher result.
If only a small roof area is connected, the tank may not receive enough runoff. If there is little reuse, the tank may remain full and have limited capacity to capture the next rainfall event.
The tank, catchment and reuse demand should therefore be considered as one treatment system.
Read Can Rainwater Tanks Improve a STORM Rating? for a focused explanation.
A raingarden can improve the result by treating runoff through filtration and biological processes before the water is discharged or infiltrated.
Its modelled contribution depends on factors such as:
A small raingarden connected to a very large driveway may not provide enough treatment. Conversely, a reasonably sized raingarden connected to a defined catchment can make a significant contribution.
Permeable paving can reduce the untreated impervious area and may provide treatment through temporary storage, filtration and infiltration.
Its effect depends on:
The modelled area should match the paving specification and drawings. Conventional paving should not be counted as permeable solely to improve the assessment result.
Two developments with similar site areas can produce very different results.
This may be due to differences in:
The score responds to the complete relationship between the development surfaces and its treatment strategy, not merely the presence of one sustainability feature.
Unexpected results are often caused by incomplete or inconsistent inputs rather than the assessment tool itself.
Common issues include:
The model inputs should be checked against the current architectural, landscape and hydraulic documentation before the report is issued.
Melbourne Water’s original STORM Calculator has been replaced by BlueFactor for suitable small-scale Victorian developments.
The two tools belong to the same practical assessment territory: estimating runoff and pollutant generation and testing the treatment measures proposed for a development.
Existing permits and older council correspondence may continue to refer to a STORM score, while a new project may receive a BlueFactor result.
The terminology and exact output format may differ, so the current council requirement should be confirmed before preparing the submission.
A STORM score is not a complete measure of every stormwater issue affecting a site.
It does not ordinarily determine:
These matters may require separate hydraulic, civil or flood-related documentation.
The calculation should use the latest architectural and landscape plans. Changes to footprints, driveways or paving can alter the score.
Small untreated areas may appear insignificant individually but can collectively affect the overall result.
The assessment should not rely on a tank, raingarden or permeable area that cannot fit within the proposed development.
Toilet, laundry or irrigation reuse included in the model should be documented in the relevant plans and specifications.
A catchment should only be connected to a treatment measure where the site levels and drainage arrangement make that connection credible.
The relevant council may request a STORM result, BlueFactor output, MUSIC model or another form of treatment assessment depending on the project.
Certified Energy can assess the proposed site areas and treatment measures and prepare a stormwater treatment result for suitable Victorian developments.
The assessment process may include:
Where the project is too large or complex for a STORM-style assessment, the need for MUSIC modelling or separate civil and hydraulic input should be identified.
Explore the STORM Assessment Knowledge Hub or send through the current plans and council request for an initial project review.
The assessment tool models runoff from the development’s impervious surfaces, applies the performance of connected treatment measures and compares the resulting pollutant reductions with the applicable best-practice objectives.
The result is affected by site areas, impervious catchments, untreated surfaces, treatment type, treatment size, connected catchments and rainwater reuse assumptions.
A result of 100% is generally used to demonstrate that the modelled strategy meets the benchmark represented by the assessment tool. Council requirements should still be confirmed for the individual project.
No. It means the modelled reductions reach the relevant best-practice benchmark. It does not mean that the development removes 100% of every pollutant.
Some outputs may indicate performance above the benchmark. The treatment strategy must still be practical, coordinated with the plans and acceptable to council.
No. Tank performance also depends on the roof catchment and regular reuse demand. Additional storage may provide limited benefit when those other inputs remain unchanged.
No. Relevant impervious areas should be included so the assessment accurately represents the proposed development.
Yes. Its contribution depends on its treatment area, connected catchment, configuration and whether runoff can physically drain to it.
It may improve the score where a genuine permeable pavement system is proposed and correctly represented in the assessment.
No. Flooding, detention, drainage capacity and discharge are separate matters that may require specialist engineering assessment.