Turn rainfall into potential tank water
One millimetre of rain over one square metre represents one litre of water before collection losses. Multiply the rainfall by the connected roof area, then apply your efficiency assumption.
Collected litres = rainfall (mm) × roof area (m²) × efficiency (%) ÷ 100
For example, 15 mm of rainfall over 200 m² of connected roof at 80% efficiency gives 2,400 litres. The 80% setting is an illustrative assumption, not a measured performance rating for your roof.
Collection is different from storage
Water can only be stored if the tank has space. A 22,500-litre tank holding 21,000 litres has 1,500 litres of headroom. If 2,400 litres reaches it before any use, 1,500 litres fits and 900 litres exceeds the available space.
Available space = capacity − water already stored
Stored rainwater = min(collected litres, available space)
Potential overflow = max(0, collected litres − available space)
Choose useful inputs
Use the horizontal catchment area that drains into the connected downpipes. Roof sections feeding another system do not count. Efficiency can vary with first-flush diversion, leaks, splash and other losses; a single percentage simplifies these effects.
This tool treats the rain as one event arriving before further household use. In real life, use during the rain can create extra storage space. The 30-day planner lets you place rainfall on a selected day alongside daily use and a possible refill.
Common questions
Is the rainfall value a forecast?
No. You enter it as a scenario. The calculator does not retrieve weather data, and it cannot guarantee how much rain will fall on your roof.
Why does more rain sometimes add no stored water?
A full tank has no remaining storage space. Additional collection is shown as potential overflow. To compare different tank sizes, change capacity while keeping the actual starting litres consistent.
Does the result mean the collected water is safe to drink?
No. This is a quantity calculation. It does not assess water quality, treatment or suitability for a particular use.