How it works
One millimetre of rain over one square metre delivers one litre, so area × rainfall gives the gross volume. Multiplying by a runoff coefficient accounts for losses to first-flush, splash, evaporation and gutter overflow.
Worked example
200 m² × 600 mm × 0.85 = 102,000 L per year — roughly 8,500 L in an average 50 mm month.
Assumptions & limitations
Uses the roof's plan (footprint) area, not the sloped surface — rain falls vertically, so footprint is correct. Real yield depends on rainfall pattern, tank size and overflow; a small tank in a big storm spills the excess.