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Rainwater Harvesting Tank Sizer
Turn your roof area and local rainfall into a realistic annual harvest volume and a sensible tank size.
The core formula
Every square metre of roof catches one litre of water for every millimetre of rainfall that falls on it, before losses. So annual harvestable volume (in litres) = roof area (m²) × annual rainfall (mm) × collection efficiency. Efficiency accounts for losses from evaporation, first-flush diversion, gutter overflow during heavy storms, and filter losses — typically 75–85% for a well-maintained system.
Sizing the tank, not just the harvest
A tank doesn't need to hold a whole year's harvest — rain falls intermittently while demand is roughly constant, so the practical sizing question is how many days of dry-period demand you want the tank to cover as a buffer. This calculator multiplies your daily demand by that buffer period to suggest a realistic tank size, and separately shows whether your roof's annual harvest can even cover your annual demand in the first place.
Frequently asked questions
Why is collection efficiency less than 100%?
Even a well-designed system loses some water to evaporation off the roof, the "first flush" of dirty runoff that's deliberately diverted away from the tank at the start of each rain event, small leaks and overflow during very heavy downpours, and filtration. 75–85% is a realistic range for a properly maintained residential system.
How is the recommended tank size worked out?
It's your daily water demand met by harvested rain multiplied by the number of dry-period buffer days you enter — the number of consecutive dry days you want the tank to be able to supply water through without any fresh rainfall. A longer dry season or less reliable rainfall pattern generally calls for more buffer days.
What if my annual harvest is less than my annual demand?
That means your roof and rainfall alone can't fully meet that level of demand year-round — you'd either need a larger catchment area, need to reduce the demand met by rainwater, or plan to supplement with mains water during drier periods. The coverage percentage shown tells you how close the harvest comes to fully meeting demand.
Does roof material affect the calculation?
Not directly in this simplified model — the collection efficiency percentage is where you'd account for a rougher or more absorbent roofing material (like some tiles) collecting slightly less water than a smooth metal roof, by entering a somewhat lower efficiency figure.