Free tool
Size a stormwater detention tank against your council's own method, and get a report you can put in front of them.
What it does
Most detention tank sizing in New Zealand is done in a spreadsheet that was copied from another spreadsheet. The method drifts, the rainfall gets stale, and the reviewer at the council ends up checking arithmetic instead of engineering.
This calculator does the same job against the territorial authority's published method, shows every step, and produces a PDF report with the working, the tank specifications and the manufacturer's datasheets attached. It is free, it needs no login, and it runs in the browser.
It is built and maintained by SAE Ltd because we needed it ourselves. The methods are transcribed from each council's own documents and checked against jobs we have already had approved.
The methods
Where a council publishes a lookup table by roof area, impervious area and lot size, the calculator reads it straight off that table. No interpretation, no substitution, and the table it used is cited in the report.
Retention depth over the contributing area, plus the difference between pre-development and post-development runoff volume over the design storm. Used where the council asks for hydrology mitigation rather than a peak-flow check.
Rational Method with HIRDS v4 rainfall across twelve storm durations. Pre-development flow from the coverage table, bypass flow deducted, orifice sized on the flow allowed through the tank, and storage routed across every duration to find the critical one. Climate change is applied once, to the rainfall, and never again downstream.
The report separates the calculated orifice from the chosen orifice: the design and the storage stay on the exact size, and the rounded size is stated separately so a drainlayer can drill a standard hole.
Where a council publishes its own calculator rather than a method, that calculator is transcribed and reproduced. Hastings District Council's two-event assessment is in there in full, checked line by line against the council's own file, with the version and the date it was verified printed in the report.
New
Upload the site plan and the roof, impervious and pervious areas printed on it are pulled off and carried into the calculator. Every figure is shown against the exact text it was read from, and the council is worked out from the address on the drawing.
It reads what is printed. It does not scale off the drawing and it does not estimate an area from the size of a shape, because an estimated area that ends up in a council submission is worse than a blank one. Where a plan gives the parts but no total, the parts are listed for you to tick and total yourself. Everything it returns is subject to confirmation by the designer.
Tanks and reporting
Sizing gives you a volume. The calculator then offers tanks that actually reach it, from 64 above-ground models and 42 below-ground systems across Bailey, Devan, Promax, Thin Tanks, Kiwi Tanks, Tanksalot, APD and others.
Architects rarely want one large vessel, so combinations are offered too: several identical tanks, or a mix of two sizes, ranked by how little capacity is wasted rather than by price. The report states what to buy, the installed capacity against the requirement, the plan area the set occupies, and the rule that the tanks must be manifolded to a single controlled outlet.
The PDF runs to eight pages: site measurements, the full working, the tank specifications, the council's design requirements and submission checklist, installation guidance and an operation and maintenance manual. The relevant manufacturer datasheets are merged onto the end automatically, from a library of 47.
Coverage
67 territorial authorities are configured. Only Napier City Council and Hastings District Council are open. The rest are greyed out and cannot be selected.
That is deliberate. A council is only opened once its method has been read out of its own current source document and the results checked against jobs that have been through consent. A calculator that quietly applies the wrong council's rules is worse than no calculator, so the others wait their turn.
If you need a council that is not yet open, email us and it will be prioritised.
Be clear about this
No login, no charge, no catch. It opens in a new tab.
