Rainwater Harvesting Calculator
Roof Area (m²)
Annual Rainfall (mm)
Efficiency (%)
Rainwater harvesting collects rooftop rain for garden irrigation, toilet flushing, or other non-potable uses. Collection efficiency is typically 80–90% accounting for losses.
- 1Harvestable volume (L) = Area (m²) × Rainfall (mm) × Efficiency
- 2Efficiency 85% is typical for clean roof with gutters
- 3Annual harvest varies greatly by location
- 4Tank sized to hold 2–4 weeks of typical use
80m² roof, 700mm annual rainfall, 85% efficiency=Annual harvest = 80 × 0.7 × 0.85 × 1,000 = 47,600 litres
| City | Annual rainfall | Harvest (80m², 85%) |
|---|---|---|
| Cardiff | 1,152mm | ~78,300 L |
| London | 601mm | ~40,900 L |
| Manchester | 804mm | ~54,700 L |
| Edinburgh | 680mm | ~46,200 L |
References
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