Rainwater Harvesting

Rainwater harvesting collects rooftop rain for garden irrigation, toilet flushing, or other non-potable uses. Collection efficiency is typically 80–90% accounting for losses

The calculator uses the standard published formula for rain harvest. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches, square metres, percentages. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: Harvestable volume (L) = Area (m²) × Rainfall (mm) × Efficiency. Each step in the calculation is shown so you can verify the result manually.