Rainwater Harvest Calculator
Rain falls on your roof. How much water is that, really? And how much can you collect and store for your garden?
I studied water cycles and biogeochemistry. That taught me that rainwater harvesting is straightforward. It is simple maths: roof area times rainfall depth equals available water.
This calculator shows you exactly how much water your roof can capture from a rainfall event. Then you can size your tank and plan your system accordingly.
Calculate Your Rainwater Potential
How This Works
Rainwater harvesting is based on a straightforward formula. Roof area (m²) multiplied by rainfall depth (mm) equals water volume.
One millimetre of rain falling on one square metre of roof produces one litre of water (if collected perfectly). So 50 mm of rain on 80 m² of roof produces 4,000 litres.
But real systems are not perfect. Some water is lost to debris on the roof, to gutter overflow, to pipe friction, and to evaporation during storage. Collection efficiency accounts for these losses. A typical system catches 75% to 85% of available water.
The calculator multiplies your roof area by rainfall by efficiency to show you realistic water collection.
Understanding Your Result
The calculator tells you how much water you can collect from a single rainfall event at your estimated efficiency.
Example: 80 m² roof, 50 mm rainfall, 80% efficiency = 3,200 litres of usable water from one rain event.
Use this number to size your storage tank. If you want to capture one rain event, size your tank to this volume. If you want to capture multiple events between garden watering, multiply accordingly.
Note: This is per rainfall event. To estimate annual water availability, multiply this result by the average number of rain events per year in your area (roughly 120 to 150 in the UK).
Important Limitations
This is an estimation based on average conditions. Real rainwater harvesting depends on many factors beyond roof area and rainfall.
✗ Does not account for roof material and pitch efficiency
✗ Does not account for gutter and downpipe design
✗ Does not account for first flush debris losses
✗ Does not account for tank overflow or seasonal patterns
✗ Does not account for water quality or treatment needs
✗ Does not account for local regulations
Check your local building regulations before installing a rainwater harvesting system. Some areas have restrictions or require permits.
Scientific Basis
Based on:
Water Management and Harvesting Standards. The basic principle that one millimetre of rain on one square metre equals one litre is well-established in water engineering. This forms the foundation of all rainwater harvesting calculations.
See UK water standards →
Australian Rainwater Harvesting Guidelines. Among the most comprehensive publicly available standards. They define collection efficiency factors and system design for rainwater capture.
Learn from rainwater harvesting guidance →