Estimates how much rainwater a roof catchment area can realistically collect from a given amount of rainfall.
How it works
The roof area is multiplied by the rainfall depth and by a collection efficiency factor that accounts for real-world losses like runoff and evaporation.
What this does not include
This does not include storage tank sizing or first-flush diversion system design — this calculator specifically estimates the raw collectable volume.
How to use this calculator
- Enter your roof’s catchment area, the rainfall amount, and a collection efficiency estimate.
Frequently asked questions
Why does 1 mm of rain over 1 m² equal exactly 1 liter?
Because a 1 mm depth of water spread across 1 square meter is a volume of 0.001 cubic meters, and a cubic meter of water is exactly 1,000 liters — so 0.001 m³ is exactly 1 liter.
Why isn’t collection efficiency 100%?
Some water is lost to evaporation, splashing off the roof, absorption by roofing material, and deliberate first-flush diversion (discarding the first, dirtiest runoff) before it reaches storage.
Does roofing material affect collection efficiency?
Yes — smooth, non-porous materials like metal roofing typically collect more efficiently than porous materials like some shingles.
