- Use the calculator to find the right rainwater tank size for your home or property.
- It considers roof area, rainfall patterns, and daily water consumption.
- Getting the size right saves money and ensures a reliable water supply.
- Too small a tank means running dry; too large wastes capital investment.
- Results are recommendations — always consult a plumber for final installation decisions.
Rainwater Tank Size Recommendation Calculator
Above the Fold
Get a simple rainwater tank size recommendation based on your target daily use and how many backup days you want. It also shows estimated monthly capture potential from your roof.
- Best for: irrigation planning, emergency backup, non‑potable household use
- Inputs: daily gallons + backup days (plus optional roof/rain)
- Outputs: recommended storage gallons
Below the Fold
How it works: Recommended storage ≈ daily use × backup days. We also estimate capture from roof area and average rainfall so you can sanity-check your storage choice.
Example: If you want 40 gal/day for 7 days, recommended storage starts around 280 gallons (before adjusting for capture and space).
FAQs
Should my tank match my monthly capture?
What else affects tank sizing?
Is rainwater potable?
How does the rainwater tank size calculator work?
The calculator uses your roof catchment area, average local rainfall, and estimated daily water usage to model how much rainwater you can collect versus how much you consume. It then recommends a tank capacity that balances supply and demand, reducing the risk of running out during dry periods.
What information do I need before using the calculator?
You will need your roof area in square metres, your location or average annual rainfall in millimetres, the number of people in your household, and an estimate of how the water will be used — for example, toilet flushing, garden irrigation, or whole-home supply. The more accurate your inputs, the better the recommendation.
Can I use this calculator for a commercial or rural property?
Yes. While default assumptions are based on residential use, you can adjust inputs such as roof area and daily consumption to reflect commercial buildings, sheds, or rural properties. For large-scale or complex systems, it is advisable to also seek advice from a licensed hydraulic engineer or water storage specialist.
People Also Ask
What size rainwater tank do I need for a family of four?
A family of four typically needs a tank between 5,000 and 10,000 litres for garden and toilet use, or up to 22,500 litres for whole-home supply, depending on local rainfall and roof size. Use the calculator above with your specific inputs to get a personalised recommendation for your household.
How much rainwater can I collect from my roof?
Multiply your roof catchment area in square metres by annual rainfall in millimetres, then apply a runoff efficiency factor of around 0.85 to account for losses. For example, a 150 m² roof in a region with 600 mm of annual rainfall can yield roughly 76,500 litres per year under typical conditions.
Is a bigger rainwater tank always better?
Not necessarily. An oversized tank costs more to purchase and install, takes up more space, and water may stagnate if turnover is slow. The ideal tank size matches your collection potential with your consumption needs. The calculator helps you find that balance rather than simply choosing the largest available option.