Rainwater System Planner
Plan a rainwater system that works through dry years.
This free Australian rainwater tank size calculator helps you plan for a suburban home, rural property, acreage or off-grid living using 30 years of historical daily rainfall.
Explore your roof catchment, tank storage and daily water demand
Start with your property
Size your system
Find a practical tank size for your roof and water use. Enter your location, available roof area and daily rainwater demand.
Why historical daily rainfall matters
Annual rainfall averages cannot show when dry spells and shortages occur. Daily historical rainfall lets the planner explore how storage and demand interact over time. Learn more about the planner.
The method
How this calculator works
This rainwater tank calculator uses historical rainfall, your available roof catchment, tank storage and daily water demand to show how a system would have performed.
- Rainfall
- Roof catchment
- Tank storage
- Daily demand
Historical rainfall
The calculator uses 31 complete years of daily Australian rainfall data: the first year warms up the tank, starting empty, and the following 30 years are scored. Storage carries forward continuously, with no reset at the end of a year.
Rainfall comes from the Queensland Government SILO / Long Paddock grid nearest the selected location. This is interpolated grid data, not a rain gauge at your property. SILO data is licensed under CC BY 4.0; the data is used here to model rainwater capture and supply.
Roof catchment and daily demand
Before losses, 1 mm of rain on 1 m² of roof yields 1 litre. Captured water = rainfall × roof area × capture efficiency. The default efficiency is 90%, allowing for capture losses.
The model uses the litres per day entered in the form. The demand estimator only helps you choose that figure; the final daily demand in the form is what the simulation uses.
The daily tank simulation
Each day, captured rain enters the tank; water above its capacity overflows. The tank then supplies that day’s demand, and any shortfall is unmet demand. Remaining water carries into the next day. This daily model does not account for the timing of rain and water use within the same day.
Two measures of reliability
Demand reliability is water supplied divided by water demanded across the scored period. It is the main percentage used for the recommendation: 95% means the tank supplied 95% of nominated demand historically. When demand is zero, it is reported as 100%.
Years with no shortage is the percentage of scored years in which all nominated demand was met without a shortage. It is different from demand reliability.
Choosing a tank
The calculator tests practical tank sizes against the same rainfall and demand, and recommends the smallest tested tank that meets your target. If the roof catchment cannot meet the target even with effectively unlimited storage, it shows a practical size around the point where larger tanks give smaller gains, when that point can be identified; otherwise it shows the best tested size. A larger tank cannot create rainfall the roof never captures. If the target is theoretically possible but no tested size meets it, larger sizes need to be tested.
Results describe how the system would have performed across the historical rainfall period. They are not a guarantee of future rainfall or future supply.