Rainwater Harvesting Yield Calculator

About this calculator

The amount of rainwater a roof can collect follows a simple physical relationship: 1 millimetre of rain falling on 1 square metre of roof produces exactly 1 litre of water, so annual yield (litres) = roof area (m²) × annual rainfall (mm) × collection efficiency. Collection efficiency is always less than 100% because some water evaporates, splashes off, or is lost to the “first flush” diverter that clears debris and bird droppings from the roof before water reaches the storage tank — typical real-world efficiencies run 75–90% depending on roof material and system design.

Homeowners, gardeners and sustainability-focused builders use this figure to size a rainwater harvesting system correctly: an undersized tank wastes potential collection during heavy rain, while an oversized one costs more than the realistic yield justifies. It’s also the number used to estimate how much mains water a harvesting system can actually displace over a year for irrigation, toilet flushing or laundry.

This calculator takes your roof area, annual rainfall and collection efficiency and returns how many litres of rainwater you could realistically harvest each year.