Rain Water Harvesting Calculator
Estimate rainwater collection potential, recommended tank size, and days of water supply from your roof area and local rainfall.
NEW ✓ Runs in your browser · Updated 2026-03-31Enter values and click Calculate Harvest to see results
How Rainwater Harvesting Works
Rainwater harvesting collects rainfall from rooftops and stores it for later use. The amount of water collected depends on your roof area, local rainfall, and the runoff coefficient of your roof surface.
Annual Harvest (litres) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient
Note: 1 mm of rain on 1 m² = 1 litre of water
Monthly Average = Annual Harvest ÷ 12
Tank Size = Monthly Average × 1 (1 month storage)
Days of Supply = Annual Harvest ÷ Daily Demand
% Demand Met = min(100, (Annual Harvest ÷ (Daily Demand × 365)) × 100)
Runoff Coefficients
The runoff coefficient indicates how much rainfall is effectively collected. Smooth, hard surfaces like metal sheets collect more water (90%), while porous surfaces like thatch or earth absorb more and collect less.
Rainwater Harvesting in India
Rainwater harvesting is mandatory in many Indian states including Tamil Nadu, Karnataka, Rajasthan, and Delhi for buildings above a certain size. It helps recharge groundwater, reduces water bills, and provides a sustainable water source during dry months.
Benefits of Rainwater Harvesting
- Reduces dependence on municipal water supply
- Recharges groundwater table
- Reduces water bills significantly
- Provides soft, chemical-free water for gardening
- Helps prevent urban flooding by reducing runoff
Related Calculators
- Area Converter — Convert between sq metres, sq feet
- Volume Converter — Convert litres, gallons, cubic metres
Frequently Asked Questions
How much rainwater can I harvest?
Annual harvest (litres) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient (0.8 for concrete/tiles). For a 100 m² roof with 1000 mm rainfall: 100 × 1000 × 0.8 = 80,000 litres per year.