Choosing the right rainwater tank can feel confusing at first. With so many sizes, shapes, and uses, it’s easy to wonder “what size water tank do I need?”
In this practical Australian guide, Great Northern Tanks will help you work out the best tank size for your home, farm, or business—without overspending or running out of water when you need it most.
Whether you’re capturing rainwater for garden use or planning a full household supply, the tank size you choose should match your property, rainfall, roof area, and how you’ll use the water.
The best water tank size depends on how you plan to use the water. In Australia, the most common tank uses include:
So, if you’re asking what size water tank do I need, the real answer is: it depends on demand and rainfall capture potential.
Quick tip: A larger tank isn’t always “better” if your roof can’t fill it often enough. That’s why sizing properly matters.
For households, tank size is usually determined by:
Small households (1–2 people)
If you’re only using rainwater for garden and outdoor use, a 2,000L to 5,000L tank often works well.
If you want to supply toilets/laundry too, you may need 5,000L to 10,000L depending on usage and rainfall.
Families (3–5 people)
For mixed indoor/outdoor use, many Australian families choose 10,000L to 22,500L tanks.
If you’re aiming for high water independence, consider moving into the 22,500L–30,000L range.
Large households or rural properties
If your property is off-grid or has limited mains water access, the right size may be 30,000L+, particularly if the tank supplies:
Want to see options by size? Visit Great Northern Tanks’ Shop to know more sizes.
Choosing the right residential water tank size depends on how you plan to use the water around your home. A small tank may be enough for garden watering, while a larger tank may be needed if you want to connect rainwater to toilets, laundry, outdoor taps, or full household supply.
For many Australian homes, a good starting point is:
If you are sizing a water tank for a residential property, roof area is just as important as household demand. A larger tank will only be useful if your roof and rainfall can fill it often enough. For example, a small roof area may not keep a large tank full, while a bigger home with wide roof catchment may be able to support much greater storage.
Placement is another key factor. Urban and suburban homes often need slimline tanks, under-eave tanks, or smaller round tanks to fit beside the house, fence, garage, or shed. Rural homes usually have more space and may be able to install larger round tanks or multiple linked tanks.
For residential water tank sizing, think about what you want the tank to do day to day. If it is mainly for the garden, a smaller setup may be enough. If you want rainwater connected to your home, it is usually worth choosing a larger size and planning the pump, filtration, overflow, and plumbing at the same time.
Garden watering can use more water than people realise—especially in warmer parts of Australia. If you’re wondering what size water tank do I need for the garden, here’s a simple guide:
Light garden watering (small yard)
Medium garden + lawn
Large gardens, hobby farms, orchards
If you regularly water lawns, veggie gardens, fruit trees, or run drip irrigation, sizing up will save you stress during dry periods.
If you’re rural and asking what size water tank do I need for a farm, you’ll need to plan for heavier demand, including:
Farm and agricultural tanks commonly range from:
Many farms also choose multiple tanks connected together, rather than one huge tank. This can provide better flexibility and backup storage.
Browse suitable options here: https://greatnortherntanks.com.au/shop/
Farm water tank sizing needs a different approach to residential sizing because demand can change quickly. Livestock, irrigation, machinery washdown, spray tanks, sheds, staff facilities, and fire reserves can all place extra pressure on stored water.
When working out what size farm water tank you need, start by listing every water use on the property. This may include:
Stock water requirements can vary depending on animal type, age, weather, feed, and pasture conditions. Hot weather can increase demand, so it is important to allow a buffer rather than sizing the tank for average use only.
Many farms use more than one tank instead of relying on a single large tank. Multiple tanks can make it easier to service different areas of the property, reduce long pipe runs, and provide backup if one tank needs maintenance. For example, a farm may use a large main storage tank, smaller header tanks near trough lines, and additional tanks near sheds or work areas.
Farm water tank costs depend on capacity, tank material, site access, base preparation, pumps, pipework, valves, trough connections, and whether multiple tanks are being installed. Larger farms may need steel panel tanks or high-capacity poly tanks, while smaller rural blocks may be suited to mid-sized round tanks.
For farm water storage, it is usually better to plan for dry conditions, not just normal rainfall. A correctly sized tank system can help reduce water stress, improve daily efficiency, and provide more confidence during seasonal changes.
In bushfire-prone regions, extra water storage can be a smart safety measure. If you’re asking what size water tank do I need for fire protection, consider:
A common recommendation is keeping at least:
Many Australians choose a dual-purpose setup: household use + reserve water that’s always kept available.
If you want the most accurate answer to what size water tank do I need, you should calculate two things:
1) How much water you use (demand)
Household water usage varies, but a rough guide can be:
For example:
A family of 4 might use 320–600L/day if rainwater is used indoors.
2) How much rain you can collect (supply)
The bigger your roof and the higher your rainfall, the more water you can harvest.
A rough guide:
1mm of rain on 1m² of roof = ~1 litre of water
So if you have a 200m² roof and you get 10mm of rain:
200m² x 10mm = 2,000L collected (approx. before losses)
A water tank size calculator can help you get a clearer idea of the tank capacity you may need before you request a quote. While every property is different, the basic calculation comes down to three things: how much water you use, how much rainwater you can collect, and how long you want your stored water to last between rainfall or refills.
As a simple starting point, consider:
A helpful rainwater harvesting estimate is:
1mm of rainfall on 1m² of roof area = about 1 litre of water collected
For example, if you have a 250m² roof area and receive 20mm of rain, you may collect around 5,000L before allowing for losses from gutters, filters, first flush systems, and overflow.
A water tank size calculator should not be treated as a final engineering answer, but it is useful for narrowing down your options. If the estimate suggests you need 8,000L, it may be worth comparing 10,000L or 13,500L tanks for extra storage. If the estimate is closer to 40,000L or more, you may need to look at larger poly tanks, steel panel tanks, or linked tank systems.
For commercial sites, farms, and rural properties, it is also worth factoring in peak demand. A business may use more water during production hours, while a farm may need more water during dry months, stock growth, or irrigation periods.
Here are some popular tank sizes Australians choose and what they’re best suited for:
2,000L – 3,000L tanks
Best for:
5,000L tanks
Best for:
10,000L tanks
Best for:
22,500L tanks
Best for:
30,000L+ tanks
Best for:
Explore the best fit for your property here: https://greatnortherntanks.com.au/shop/
When choosing the right tank size, here are mistakes that can cost you money or reduce performance:
Choosing a tank too small
A small tank may empty quickly, especially during hot Australian summers. You’ll rely more on mains water (if available), reducing your savings.
Choosing a tank too large without enough roof area
If your roof catchment is limited, a large tank can take too long to fill—and may never reach capacity.
Not planning for future water use
Families grow, gardens expand, and properties change. If you might add:
Water storage needs are very different between coastal areas and inland regions. The right solution accounts for seasonal rainfall.
If you still feel unsure about what size water tank do I need, here are simple starting points:
At Great Northern Tanks, we help Australians choose the right solution based on:
Whether you’re buying for your home, acreage, farm, or business, we make it easy to match your needs with the right tank size.
Browse available tank options here: https://greatnortherntanks.com.au/shop/
The best water tank size is the one that matches your water demand and your ability to collect enough rainwater to keep it filled.
If you want a reliable, Australian-friendly way to decide, start with:
Then confirm your choice using the Great Northern Tanks sizing tools and tank categories.
Explore Great Northern Tanks online and get started here: https://greatnortherntanks.com.au/shop/
