Size a rainwater harvesting system from roof area and rainfall.
One millimetre of rain on one square metre yields one litre, so harvest scales directly with roof area and rainfall. Tank size is determined by how long the system must bridge without rain, not by the annual total. Oversizing the tank wastes capital that never fills, while undersizing means overflow during storms, so the dry period is the parameter that matters.
Rainwater Harvesting
Harvest = roof area × rainfall × runoff coefficient; one millimetre on one square metre is one litre
Harvest = roof area × rainfall × runoff coefficient; one millimetre on one square metre is one litre One millimetre of rain on one square metre yields one litre, so harvest scales directly with roof area and rainfall. Tank size is determined by how long the system must bridge without rain, not by the annual total.
Oversizing the tank wastes capital that never fills, while undersizing means overflow during storms, so the dry period is the parameter that matters.
This calculator takes 5 inputs: Roof catchment area, Annual rainfall, Runoff coefficient, Daily water demand, Longest dry period. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.