A rooftop or paved catchment area can supply meaningful supplemental water for irrigation or livestock use, but the actual collectible volume depends heavily on rainfall pattern and collection system efficiency, not just total annual rainfall. This calculator estimates your realistic annual harvest and compares it against your current water cost to show potential savings.
Annual Harvest (liters) = Catchment Area (sq ft) × Average Annual Rainfall (inches) × 0.623 × Collection Efficiency. The 0.623 conversion factor accounts for the standard relationship between rainfall depth and volume collected per square foot of catchment.
| Catchment Type | Typical Efficiency |
|---|---|
| Metal roofing | 85-95% |
| Concrete/tile roofing | 75-85% |
| Paved ground catchment | 60-75% |
For a 2,000 sq ft metal roof catchment in an area receiving 25 inches of annual rainfall, at 90% collection efficiency: Annual harvest = 2,000 × 25 × 0.623 × 0.90 ≈ 28,035 liters per year. At a current water cost of PKR 15 per 1,000 liters, this represents a modest annual saving of roughly PKR 420 — useful as a supplemental source, though the main value in water-scarce areas is often having an emergency reserve rather than large-scale cost savings alone.
It depends on roof area, material, and local rainfall, but as a rule of thumb, every 1,000 sq ft of metal roofing in a 25-inch rainfall area can collect roughly 14,000 liters per year at typical efficiency — useful for livestock water or supplemental irrigation of a small kitchen garden.
Some rainfall is lost to evaporation, initial roof wetting before runoff begins, gutter overflow during intense storms, and small leaks in the collection system — well-maintained metal roof systems achieve the highest efficiency among common catchment types.
In low-rainfall areas, the cost-saving case is weaker, but rainwater harvesting still provides a valuable buffer during the rainy season itself and can reduce dependence on groundwater pumping during that window, even if it doesn't meet the bulk of annual water need.
For irrigation and livestock watering, minimal treatment is typically needed beyond basic filtering to remove debris — first-flush diverters that discard the initial dirty runoff from each rain event significantly improve water quality for storage.