What is a stormwater detention tank?

A stormwater detention tank temporarily stores rainwater runoff during heavy rainfall and slowly releases it at a controlled rate through a sized orifice outlet. This reduces peak discharge to the council stormwater network, achieving hydraulic neutrality.

Do I need a detention tank in New Zealand?

Most NZ councils require stormwater detention for new developments and additions that increase impervious area. Requirements vary by council. This calculator covers all 67 NZ territorial authorities with their specific rules and sizing tables.

How do I size a detention tank?

Select your council, enter your roof area, impervious area, and total site area. The calculator looks up the correct sizing table for your council and returns the required detention volume, retention volume, orifice diameter, and recommended tanks.

What is hydraulic neutrality?

Hydraulic neutrality means the stormwater runoff from your developed site does not exceed the pre-development runoff rate. A detention tank achieves this by storing excess runoff and releasing it slowly through an orifice plate.

Which councils are covered by this calculator?

All 67 NZ territorial authorities are covered, including Auckland, Wellington, Christchurch, Hamilton, Napier, Hastings, Tauranga, Dunedin, Queenstown, and all regional councils. Each has council-specific sizing rules, design requirements, and submission checklists.

NZ Detention Tank Calculator | Free Stormwater Sizing for 67 Councils

Select Your Council

Territorial Authority

Napier City Council — Stormwater Standard Design Guidance v2 (Dec 2025)
Submit to: engineeringapprovals@napier.govt.nz

Project / Lot Details

Lot / Project Number

Site Address

Developer / Calculated By

Contact Email

Step 1

Total Site Area (A_t)

Measure the total area of the site (lot).

Total Site Area (m²)

Step 2

Total Roof Area (A_r)

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.

Roof Area (m²)

Step 3

Total Impervious Area (A_i)

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.

Impervious Area (m²)

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.

Existing Impervious (m²) Before the proposed work

New Impervious (m²) Being added by this work

Total: 0 m² | This breakdown is included in the PDF report for the council reviewer.

Step 4

Retention Volume (R_v)

Retention volume is calculated from roof area and impervious area. This is the water harvested for reuse (garden watering, car washing, toilets, laundry).

R_v = (A_r + A_i) × 10

A_r (Roof Area)

0 m²

A_i (Impervious Area)

0 m²

R_v = 0 litres

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.

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.

Rainfall

HIRDS Rainfall Intensity Data

Select a location to auto-fill rainfall data, or enter manually from HIRDS (hirds.niwa.co.nz).

Location (HIRDS Data)

Design ARI

Climate Change Included?

Tip: Select a location and click Auto-Fill, or enter values manually below. Data source: HIRDS v4 (NIWA). For exact site data, query hirds.niwa.co.nz directly.

Duration (min) Intensity (mm/hr)

10 min

20 min

30 min

40 min

50 min

60 min

120 min

180 min

240 min

360 min

540 min

720 min

Pre-Dev

Pre-Development Coverage

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

Pre-Development Time of Concentration, Tc (min) 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

Orifice Head Height — H (m) Height from orifice invert to overflow invert

Number of Orifices

Results

Stormwater Management Summary

Acceptable Solution

Flow Analysis

Retention Volume (R_v)

R_v = (A_r + A_i) × 10

Detention / Storage Volume

Orifice Plate

Total Minimum Tank Volume

Inflow / Outflow / Storage Analysis

Tank Proportions

Recommended Tanks

Enter your own tank details. Use this if you have a specific tank in mind or a custom-built solution.

Tank Purpose

Tank Shape

Manufacturer

Model / Reference

Diameter (mm)

Height (mm)

Capacity (L) auto-calculated or override

Outlet Size (mm)

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)

Tank Make

Tank Model

Tank Capacity (L)

Dimensions

+ Add Custom Tank

Click to expand

Tank Purpose

Tank Shape

Manufacturer

Model / Reference

Diameter (mm)

Height (mm)

Capacity (L) — auto-calculated or override

Outlet Size (mm)

Note: Attach manufacturer's specification sheet with your submission.

NZ Detention Tank Calculator

Select Your Council

Project / Lot Details

Total Site Area (A_t)

Total Roof Area (A_r)

Total Impervious Area (A_i)

Retention Volume (R_v)

Run-off Coefficients & Slope Correction

HIRDS Rainfall Intensity Data

Pre-Development Coverage

Post-Development — Flows to Tank

Post-Development — Lost Flows (Not Connected to Tank)

Orifice Configuration

Stormwater Management Summary

Acceptable Solution

Flow Analysis

Retention Volume (R_v)

Detention / Storage Volume

Orifice Plate

Total Minimum Tank Volume

Inflow / Outflow / Storage Analysis

Tank Proportions

Recommended Tanks

Design Requirements

Reference: Sizing Table

Tank Selection

Combined Tank (Retention + Detention)

Retention Tank

Detention Tank

+ Add Custom Tank

Submission Checklist

Required Documents

Need Professional Engineering?

Stay Updated

Support This Tool

Feedback

How to Use This Calculator

Tips

Need Help?

Select Your Council

Project / Lot Details

Total Site Area (At)

Total Roof Area (Ar)

Total Impervious Area (Ai)

Retention Volume (Rv)

Run-off Coefficients & Slope Correction

HIRDS Rainfall Intensity Data

Pre-Development Coverage

Post-Development — Flows to Tank

Post-Development — Lost Flows (Not Connected to Tank)

Orifice Configuration

Stormwater Management Summary

Acceptable Solution

Flow Analysis

Retention Volume (Rv)

Detention / Storage Volume

Orifice Plate

Total Minimum Tank Volume

Inflow / Outflow / Storage Analysis

Tank Proportions

Recommended Tanks

Design Requirements

Reference: Sizing Table

Tank Selection

Combined Tank (Retention + Detention)

Retention Tank

Detention Tank

+ Add Custom Tank

Submission Checklist

Required Documents

Need Professional Engineering?

Stay Updated

Support This Tool

Feedback

How to Use This Calculator

Tips

Need Help?

Total Site Area (A_t)

Total Roof Area (A_r)

Total Impervious Area (A_i)

Retention Volume (R_v)

Retention Volume (R_v)