1. How Rainwater Harvesting Works
A rainwater harvesting system collects water that falls on a roof, directs it through gutters and downpipes, filters or screens debris and stores the water in a rainwater tank for later use.
A typical system is: Roof → Gutters → Downpipes → Strainer / First Flush → Rainwater Tank → Pump or Gravity Supply.
Depending on system design and local requirements, stored rainwater can be used for garden irrigation, washing vehicles and equipment, outdoor cleaning, toilet flushing, laundry, pools, rural applications and other uses.
The key is not simply buying the largest tank that fits. Good tank sizing matches roof catchment, rainfall, available space and water demand.
2. How Much Rainwater Can You Collect?
Rainwater collection is straightforward to estimate. One millimetre of rain falling on one square metre of roof represents approximately one litre of water before losses.
Example: 200 m² roof and 25 mm of rain
Using 90% collection efficiency: 200 × 25 × 0.90 = 4,500 litres.
With 50 mm of rain the same roof could collect approximately 9,000 litres.
3. Annual Rainwater Harvesting Potential
The same calculation can estimate annual collection. If a 200 m² connected roof receives 1,000 mm of rainfall over a year:
That does not mean the property needs a 180,000 litre tank. Water arrives throughout the year and is also being used. Storage capacity is intended to capture useful rainfall when it occurs and provide water between rainfall events.
4. What Size Rainwater Tank Do I Need?
Four factors matter most:
Roof catchment area
A larger connected roof can supply more water to the tank.
Local rainfall
Total annual rainfall matters, but so does timing. A location with long dry periods may benefit from more storage than a location receiving similar annual rainfall spread evenly through the year.
Water usage
A garden-only system generally requires less storage than a system supplying substantial irrigation, toilets, laundry or rural use.
Available space and access
The tank must physically fit the site and delivery path, with room for a suitable base, inlet, outlet, overflow, plumbing and maintenance access.
| Application | Storage Capacity to Consider |
|---|---|
| Small garden / supplementary water | 1,000–3,000 L |
| Urban home with limited space | 2,000–5,000 L |
| Garden + outdoor household use | 5,000–10,000 L |
| Larger suburban property | 10,000–20,000 L |
| Large roof / acreage / substantial storage | 20,000–30,000 L |
| High water demand | Multiple tanks may be appropriate |
These figures are a starting point rather than a substitute for calculating the needs of an individual property.
5. Choosing the Right Type of Water Tank
Round Water Tanks
Where sufficient ground space is available, PTQ round water tanks provide a wide choice of capacities from 340 litres to 30,000 litres. Round tanks are suited to homes, acreage, sheds, workshops, farms, commercial properties and large roof catchments.
PTQ manufactures round tanks in Brisbane from food-grade, UV-stabilised polyethylene. The range includes commonly used capacities such as 5,000 L, 10,000 L, 20,000 L, 22,500 L and 30,000 L.
See all PTQ round rainwater tank sizes and specifications →
Slimline Water Tanks
Where a conventional round tank will not fit, PTQ slimline water tanks are designed for narrow spaces beside houses, fences, under eaves and between structures.
PTQ's slimline range runs from 1,280 litres to 5,080 litres, with selected models only 565 mm wide. This makes slimline tanks particularly useful for urban properties where side access and yard space are limited.
Compare PTQ slimline water tank sizes →
Underdeck Water Tanks
Elevated homes and decks can provide valuable storage space. PTQ underdeck water tanks are low-profile polyethylene tanks designed for locations where conventional upright tanks cannot fit.
PTQ manufactures 1,000 L, 1,500 L and 2,000 L underdeck tanks, with heights starting at approximately 294 mm.
6. One Large Tank or Several Smaller Tanks?
One large tank
A single large tank can mean fewer plumbing connections, fewer outlets and overflows, a simpler installation and potentially lower cost per litre of storage.
Multiple tanks
Several smaller tanks can make sense when space is divided around the property, delivery access is restricted, different roof sections need to be harvested or future expansion is planned.
Multiple tanks can be interconnected so they operate as a larger storage system, provided the plumbing, tank levels and overflow arrangement are correctly designed.
7. How Quickly Will a Rainwater Tank Fill?
Consider a 250 m² connected roof.
100 mm rainfall: 250 × 100 × 0.90 = 22,500 litres
This explains why large-capacity storage can make sense on properties with substantial roof catchment. A small tank may fill early in a storm, with additional water then leaving through the overflow.
If your property has substantial catchment, compare PTQ's large round rainwater tanks up to 30,000 litres.
8. Calculate the Roof Area Actually Connected to the Tank
Total roof area and connected roof area are not always the same. A 250 m² roof does not provide a 250 m² catchment if only half of the downpipes are directed to the tank.
Before increasing tank capacity, check whether additional roof sections can practically be connected. Improving the connected catchment can sometimes increase harvesting more effectively than simply adding storage.
9. Choosing the Tank Location
Measure and check all of the following before ordering:
- Available width or diameter at the final position.
- Available height, including room for fittings and pipework.
- Delivery access through gates, paths and around buildings, fences, trees and services.
- Distance from connected downpipes.
- Pump position, power supply and maintenance access where a pump is used.
- Overflow drainage once the tank is full.
10. Tank Base Preparation
A full tank is extremely heavy. One litre of water weighs approximately one kilogram.
| Stored Water | Approximate Water Weight |
|---|---|
| 5,000 L | 5 tonnes |
| 10,000 L | 10 tonnes |
| 22,500 L | 22.5 tonnes |
| 30,000 L | 30 tonnes |
The tank therefore needs a suitable, level and correctly prepared foundation that supports the tank as required. Follow the manufacturer's installation requirements for the specific tank.
11. Tank Overflow Is Part of the System
Once a tank is full, incoming rainwater must leave through the overflow. During heavy rainfall this can be a large flow. The overflow should be appropriately sized and directed so it does not undermine the tank base, flood buildings, cause erosion or create drainage problems.
For PTQ tank fitting arrangements, see PTQ tank fitout information.
12. Keeping Collected Rainwater Cleaner
Good rainwater harvesting starts before the water reaches the tank. Common measures include:
- Keeping gutters reasonably clear of leaves and debris.
- Using leaf screens or guards where appropriate.
- Using a suitable tank inlet strainer.
- Using a first-flush device where appropriate.
- Keeping stored water protected from light to help limit algae growth.
- Screening openings and overflows appropriately.
13. Do I Need a Rainwater Pump?
Gravity supply can work where the outlet is above the point of use and only modest pressure is needed. Many household systems use a pressure pump for garden hoses, irrigation, outdoor taps, toilets, washing machines or other connected fixtures.
Pump selection should be based on required flow and pressure, pipework, distance and elevation, rather than motor power alone.
PTQ stocks rainwater-related products and pumps through the PTQ online store.
14. Can Rainwater Be Used Inside the House?
Rainwater systems can be designed to supply selected household fixtures, subject to applicable plumbing, health and regulatory requirements. Requirements become more significant when water is intended for potable use.
Water quality depends on the whole system: roof, gutters, screens, first flush, storage, plumbing, maintenance and any filtration or treatment.
15. Why Use a Polyethylene Rainwater Tank?
PTQ manufactures its water tanks in Brisbane using rotational moulding. PTQ round and slimline tanks are made from food-grade, UV-stabilised polyethylene and are listed by PTQ as meeting AS2070 and AS4020 material requirements. The tanks are one-piece rotomoulded products and carry a 10-year manufacturer's guarantee.
You can compare the complete range in the Plastic Tanks QLD online store.
16. Worked Example: Brisbane Suburban Home
Assume a connected roof catchment of 180 m² and collection efficiency of 90%.
75 mm rainfall: 180 × 75 × 0.90 = 12,150 litres
A 5,000 litre tank could collect most of the first event if it were empty, but would fill well before the theoretical collection from the larger event. If water is regularly being used, additional storage may allow the property to retain more of the available rainfall.
17. Worked Example: Large Shed or Acreage Property
Now assume a 400 m² connected roof at 90% collection efficiency.
100 mm rainfall: 400 × 100 × 0.90 = 36,000 litres
On a catchment of this size, a small tank can fill quickly. Larger 20,000 L, 22,500 L and 30,000 L round tanks or multiple interconnected tanks may be worth considering.
18. A Simple Way to Choose Your Tank
Write down these five figures before ordering:
- Connected roof catchment: _____ m²
- Approximate local rainfall: _____ mm/year
- Expected water usage: _____ L/day
- Maximum tank width or diameter: _____ mm
- Maximum tank height: _____ mm
Start with the roof and the water demand, not the tank catalogue. Once you understand how much water you can collect, how much you will use and how much physical space is available, choosing the tank becomes much easier.
Plenty of room? Start with round water tanks. Narrow side access? Compare slimline water tanks. Low unused space under a deck or elevated home? Look at underdeck water tanks.
19. Plastic Tanks QLD Rainwater Tank Range
Plastic Tanks QLD manufactures polyethylene water tanks at Carole Park, Brisbane. The current PTQ range includes:
- Round tanks: 340 L to 30,000 L.
- Slimline tanks: 1,280 L to 5,080 L.
- Underdeck tanks: 1,000 L, 1,500 L and 2,000 L.
PTQ lists its water tanks as manufactured from food-grade, UV-stabilised polyethylene and backed by a 10-year manufacturer's guarantee.
Need help choosing a rainwater tank?
Compare the full Plastic Tanks QLD product range or contact PTQ to discuss tank capacity, dimensions, access and delivery.
Phone: (07) 3271 1305
Email: sales@ptq.com.au
20. Rainwater Harvesting FAQs
What size rainwater tank should I buy?
Tank size depends on connected roof area, rainfall, water usage and available space. Many suburban properties consider 5,000–10,000 litres, while larger properties and sheds can often make effective use of 20,000 litres or more.
How much water does 10 mm of rain produce?
Each square metre of roof receiving 10 mm of rain receives approximately 10 litres. A 200 m² connected roof therefore receives about 2,000 litres before allowing for collection losses.
How much rainwater can I collect from a 200 m² roof?
For every 10 mm of rainfall, the theoretical maximum is approximately 2,000 litres. At 90% collection efficiency, this becomes approximately 1,800 litres.
Is a bigger rainwater tank always better?
No. A larger tank is useful when the roof can supply enough water and water is being used often enough to create capacity before the next rainfall event. The best result comes from matching catchment, rainfall, usage and storage.
Are round tanks better than slimline tanks?
Round tanks are generally preferable where space is available and storage capacity is the priority. Slimline tanks are intended for locations where width is restricted.
Can several water tanks be connected together?
Yes. Multiple tanks can be interconnected to increase storage capacity, provided the foundations, tank levels, pipework and overflows are correctly designed and installed.
How heavy is a full rainwater tank?
One litre of water weighs approximately one kilogram. A 5,000 litre tank contains about 5 tonnes of water; a 22,500 litre tank contains about 22.5 tonnes.