Measure the total roof area which shall be connected to the tank, including the eaves.
Tip: Maximise roof area connected to the tank to increase water capture and reduce over-detention required by impervious surfaces bypassing the tank.
Step 3
Total Impervious Area (Ai)
Measure the total sealed impervious areas not connected to the tank — driveways, patios, paths and other surfaces which are unable to absorb water and drain to the council's stormwater network.
Addition or alteration? Break down existing vs new impervious
For additions or alterations to existing dwellings. Some councils (e.g. Auckland SMAF) size detention based on the new impervious area only. For Napier and most councils, total impervious is used regardless.
Before the proposed work
Being added by this work
Total: 0 m²
| This breakdown is included in the PDF report for the council reviewer.
Step 4
Retention Volume (Rv)
Retention volume is calculated from roof area and impervious area. This is the water harvested for reuse (garden watering, car washing, toilets, laundry).
Rv = (Ar + Ai) × 10
0 m²
0 m²
Rv = 0 litres
Method
HDC On-Site Detention
Hastings District Council assesses detention against a maximum runoff coefficient at
two design events. Storage is required when the site's equivalent C exceeds the
maximum for its zone at either the 5-year or the 50-year storm.
Site
Site Details
Garage and driveway are entered as bands because the council's calculator does the same; each band
uses its upper figure. Remaining area is treated as garden or grass.
Storage
Storage & Outlet
Must be at least the minimum the calculation returns. Leave blank to use that minimum.
Tank height available to the stored volume. Sets the orifice size.
The council's calculator uses 10 minutes.
Method
Hydrology Mitigation by Volume
Napier City Council District Plan standard SW-S1. Retention is a fixed depth over the
impervious area; detention is the difference between pre- and post-development runoff volumes for the
10% AEP 24-hour event, less the retention achieved, drained down over 24 hours.
SW-S1 in full:
(a) Retention of at least 10 mm runoff depth over the impervious area for which hydrology mitigation is required.
(b) Detention with a 24-hour drain-down for the difference between pre- and post-development runoff volumes from the
10% AEP, 24-hour rainfall event on HIRDS RCP8.5 2081-2100, minus the retention volume achieved.
(f) The impervious area requiring mitigation is any new or redeveloped impervious area since the rule became operative.
This route typically gives a much larger tank than the acceptable-solution table or a
limiting-discharge design on the same site. That is what the standard asks for as written. Compare all
three before committing, and confirm the basis with NCC on anything marginal.
Rainfall
Design Rainfall Depth
A depth, not an intensity. SW-S1 sizes on volume.
Pre-Dev
Pre-Development Coverage
The site as it exists before this development. Runoff volume = C × Area × depth.
Surface Type
C
Area (m²)
CA
Existing Roof
0
Existing Paving
0
Existing Permeable
0
TOTAL
0
0
Post-Dev
Post-Development Coverage
The whole site after development, whether or not it drains to the tank. SW-S1 compares total site runoff volumes.
Surface Type
C
Area (m²)
CA
Roof
0
Paving / Hardstand
0
Permeable
0
TOTAL
0
0
SW-S1(f): new or redeveloped impervious area only. Sets the retention volume. Leave blank to use all post-development impervious area.
SW-S1(a) minimum is 10 mm. A greater achieved retention reduces the detention required.
Outlet
Outlet & Drain-Down Check
SW-S1(b) sets the detention volume and a 24-hour drain-down. It does not set a discharge rate,
so the orifice is yours to choose — usually from whatever discharge limit applies to the site
(consent notice, kerb capacity, or pre-development matching under the Advanced mode). Enter the size you
intend and the drain-down time is reported as a check.
The size you intend to drill. SW-S1 is a volume standard, so it does not fix the orifice; drain-down is reported below as a check.
Height from orifice invert to overflow invert.
E1/VM1
Run-off Coefficients & Slope Correction
Reference table from NZ Building Code E1/VM1. Use these C values when entering areas below. Adjust for site slope.
Surface Type
Base C
Slope Adj.
Adjusted C
Note: Base C values assume average slope (5–10%). C values are capped at 0.95 max and 0.05 min after adjustment.
Applied to the intensities below when you Auto-Fill. Nothing downstream is scaled again.
Tip: Select a location and click Auto-Fill, or enter values manually below. Data source: HIRDS v4 (NIWA). For exact site data, query NIWA HIRDS directly and type the values in. Climate change is applied once, here. The intensities in the grid are the design rainfall — every flow, the orifice and the storage are all derived from them. If you paste values straight from a HIRDS RCP8.5 query, leave the scenario alone: the calculator will not scale them again.
Duration (min)Intensity (mm/hr)
Allowable
Allowable Discharge from the Site
The pre-development flow is calculated from the coverage table below. The chosen outflow is what you
design to — usually the pre-development flow, or a lower figure set by a consent notice or the
council. The flow allowed through the tank is the chosen outflow less the bypass (lost) flow.
Carried into the report as the design basis. Kerb outlets are commonly 4 L/s single, 8 L/s double.
Pre-Dev
Pre-Development Coverage
Checked against the pre-dev total and against captured + lost. Leave blank to skip the check.
Enter the existing site coverage before development. Use runoff coefficients: Roof=0.95, Paving=0.85, Permeable=0.30
Surface Type
C
Area (m²)
CA
Existing Roof
0
Existing Paving
0
Existing Permeable
0
TOTAL
0
0
Overland flow time from furthest point. Min 10min for most councils.
Post-Dev
Post-Development — Flows to Tank
Enter areas that will be connected to the detention tank after development.
Surface Type
C
Area (m²)
CA
Captured Roof
0
Captured Paving
0
Captured Permeable
0
TOTAL
0
0
Lost
Post-Development — Lost Flows (Not Connected to Tank)
Enter areas that will NOT be connected to the tank (bypass flows to council network).
Surface Type
C
Area (m²)
CA
Unconnected Roof
0
Unconnected Paving
0
Unconnected Permeable
0
TOTAL
0
0
Orifice
Orifice Configuration
Height from orifice invert to overflow invert. Defaults to 2.0 m, or the selected council's figure.
A standard hole the drainlayer can drill — 16 mm becomes 15, 18 mm becomes 20. This does not change the calculation; storage and the design basis stay on the calculated orifice. Rounding up passes more flow, which is checked against the allowable discharge.
Set the size you intend to drill. Rounding up passes more than the design allows and is flagged.
Results
Stormwater Management Summary
Acceptable Solution
Flow Analysis
Retention Volume (Rv)
Rv = (Ar + Ai) × 10
Detention / Storage Volume
Orifice Plate
Total Minimum Tank Volume
HDC Detention Assessment
SW-S1 Volume Calculation
Inflow / Outflow / Storage Analysis
Tank Proportions
Recommended Tanks
Below ground tanks require site-specific design by a suitably qualified person. Some councils (e.g. Napier) only accept above-ground tanks under their acceptable solutions.
NZ Underground Tank Suppliers:
Loading underground tank database...
Enter your own tank details. Use this if you have a specific tank in mind or a custom-built solution.
Design Requirements
Reference: Sizing Table
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Tank
Tank Selection
Select the desired tank make and model using the retention & detention volumes calculated above.
Combined Tank (Retention + Detention)
Retention Tank
Detention Tank
+ Add Custom Tank
Click to expand
Note: Attach manufacturer's specification sheet with your submission.
Submit
Submission Checklist
Required Documents
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Need a Site-Specific Design?
If your site is outside the acceptable solutions, needs CPEng sign-off, or requires a full stormwater assessment, we can help. SAE Ltd provides professional detention tank design for any council in NZ.
Sites outside acceptable solution limits
Multi-lot subdivisions and commercial developments
CPEng-signed stormwater assessments
Council submission packages
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How to Use This Calculator
Step 1: Select your council from the dropdown. The calculator automatically loads the correct rules and tables for your area.
Step 2: Choose Simple Mode for standard residential (if your council has lookup tables) or Advanced Mode for site-specific design using the Rational Method.
Step 3: Enter your site details — total site area, roof area, and impervious area (driveways, paths, patios not connected to the tank).
Step 4: Click Calculate to get your retention volume, detention volume, orifice size, and recommended tank options.
Step 5: Select a tank from the recommendations (or add a custom tank), then use the submission checklist and PDF report for your council application.
Tips
Maximise roof area connected to the tank to reduce over-detention
Use the Lookup Rainfall button to auto-fill HIRDS data for your address
Save your project with the Save button to continue later
The PDF report is formatted for council submission
Switch between Above Ground and Below Ground tank recommendations