Rainwater Harvesting Calculator — Annual Collection
Calculate annual rainwater collection potential from your roof.
How We Calculate This
Annual collection = Roof Area × Annual Rainfall × Runoff Coefficient.
Runoff coefficients: Pitched roof 0.85, Flat roof 0.6, Green roof 0.3.
Regional rainfall: SE England ~600mm, Midlands ~750mm, NW England ~1000mm, Scotland ~1200mm, Wales ~1300mm.
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.