Free Rainwater Tank Size calculator with clear step-by-step results.
Two numbers size a rainwater system: annual yield, which is roof plan area times rainfall times a runoff coefficient, and storage, which is set by the longest dry period you need to ride out. A tank larger than the dry-period requirement adds cost without adding usable water.
Annual yield
Litres = roof plan area m2 x rainfall mm x runoff coefficient, less first-flush losses
Storage requirement
Litres = daily demand x longest dry period in days
Rain falls vertically, so the catchment is the horizontal footprint. A steep roof catches no more rain than its plan area regardless of how much surface it has.
Around 0.9 for metal roofing, 0.8-0.85 for tile and 0.7 or less for porous surfaces. Losses come from wetting, evaporation and gutter overshoot.