Rainwater Tank Days Supply Calculator
Estimate Australian rainwater tank supply days from capacity, starting level, reserve level and measured daily demand.
Your results
Calculation breakdown
- Days supply
- capacity × (starting fraction − reserve fraction) ÷ daily demand
Worked example
A 5,000 L tank starting at 80% with a 10% reserve has 3,500 L available, equal to 14 days at 250 L daily demand.
Assumptions
- No rainfall enters during the supply period.
- Daily demand and usable capacity remain constant.
Sources
Rates effective 23 August 2026.
How this calculator works
Estimate days of rainwater supply above a chosen reserve from tank capacity, starting fill percentage and average daily demand. The reserve is retained explicitly so nominal tank volume is not mistaken for water available to consume. The calculation exposes its units and intermediate assumptions so Australian users can reconcile the estimate with bills, meters, labels, design records or site measurements.
Frequently asked questions
What should I measure for the rainwater tank days supply result?
Use a measured current tank level, usable tank capacity and demand from the connected fixtures or irrigation schedule. Keep the measurement period and unit basis consistent across every input.
What can make this rainwater tank days supply estimate differ?
Future rain, pump cut-out level, sediment volume, leaks, variable demand, evaporation and water-quality restrictions are excluded. Recalculate with measured operating data before making a purchase, design or reporting decision.
Does this rainwater tank days supply calculator confirm compliance?
No. It provides a transparent planning calculation only. For this topic, use a measured current tank level, usable tank capacity and demand from the connected fixtures or irrigation schedule. Obtain qualified advice where electrical, plumbing, building, safety or reporting obligations apply.
Actual prices, usage, efficiency and savings vary by provider, equipment, location and behaviour.