Rainwater Tank Size Calculator Australia

How to Size a Rainwater Tank in Australia

The biggest mistake people make when choosing a tank is buying a massive storage vessel that their roof can never fill, or fitting a small slimline unit that empties after two days of garden watering. To get the size right, you need to calculate two numbers:

1. Harvest Capacity (Annual Roof Yield): How much rainwater your roof actually catches during rain events each year.
2. Water Demand: How much water your connected appliances (toilets, laundry, garden taps) draw per day.

The Roof Runoff Formula

The annual volume of water your roof collects is calculated using standard hydraulic catchment math:

Not all roof materials shed water equally. Some absorb initial rainfall before runoff reaches the gutters:

• Colorbond / Metal Roofs (0.90 efficiency): Smooth metal sheds almost all rain immediately. 90% of rain reaches the tank.
• Concrete Roof Tiles (0.85 efficiency): Slightly porous. Tiles absorb the first 1–2mm of rain during dry spells.
• Terracotta Roof Tiles (0.80 efficiency): More porous than concrete, leading to slightly higher absorption losses.

Average Annual Rainfall & Harvest Yields Across Australia

Rainfall patterns across Australia vary significantly. While Sydney gets over 1,200mm annually in intermittent heavy storm events, Melbourne gets around 650mm spread more evenly across the year. The table below shows typical annual harvest yields per 100m² of roof plan area based on BOM historical averages.

State Building Regulations & BASIX Rules

Depending on where you are building in Australia, rainwater tanks are often a mandatory requirement:

• New South Wales (BASIX): Under NSW BASIX building approvals, new homes and major renovations usually need a minimum 2,000L to 3,000L rainwater tank connected to all toilets, washing machine cold taps, and at least one outdoor hose tap to hit the required 40% water reduction score.
• Victoria & Queensland: Local councils often require a 2,000L to 5,000L rainwater tank or a solar hot water system for new residential builds under 6-star energy compliance codes.
• South Australia & Western Australia: Mains water top-up systems or dedicated rainwater storage are recommended for suburban lots, while semi-rural properties without mains supply require dedicated fire-fighting reserves (typically 10,000L to 22,000L with Storz fittings).

Worked Example — Sizing a Tank for a 4-Bedroom Home in Sydney

Essential Plumbing Accessories for Rainwater Systems

Installing the tank is only half the job. Under AS/NZS 3500.1 (Water Services), rainwater harvesting systems connected to household plumbing require:

• First-Flush Diverter: Captures the initial 20 to 40 litres of roof wash carrying dust, leaves, and bird droppings so it never enters the main tank storage.
• Leaf Strainer Basket & Mesh Screen: Prevents mosquitoes, leaves, and debris from contaminating the water.
• Automatic Mains Changeover Valve (Rain-to-Mains Switch): Automatically switches connected toilets and laundry to mains town water when the tank runs empty during dry spells, ensuring uninterrupted supply.
• Backflow Prevention (AS/NZS 3500.1): Dual check valves or air gaps must be fitted to prevent tank water from back-siphoning into the municipal town water main.

Common Rainwater Tank Types & Standard Sizes

• Slimline Poly Tanks (2,000L – 5,000L): Narrow width (550mm to 800mm) designed to fit down standard side passages alongside fence lines on narrow suburban blocks.
• Round Poly Tanks (3,000L – 22,500L): The most cost-effective per litre. Durable UV-stabilised food-grade polyethylene, ideal for backyards, sheds, and rural blocks.
• Underground Storage Tanks (3,000L – 10,000L): Buried underground beneath driveways or lawns where surface space is tightly constrained. Requires a submersible pressure pump.