Rainwater Harvest Calculator
Calculate how much rainwater you can harvest from your roof, determine optimal tank size, and estimate water savings.
Roof & Rainfall Details
India average: 600-2500mm. Mumbai: 2400mm, Delhi: 800mm, Bangalore: 970mm
Accounts for losses due to evaporation, absorption, and spillage
How Rainwater Harvesting Works
Collection Formula
Rainwater Collection (liters) = Roof Area (m²) × Rainfall (mm) × Runoff Coefficient
Example: 150 m² roof × 1200mm rainfall × 0.85 coefficient = 153,000 liters/year
Runoff Coefficients by Surface
- Metal Sheet (0.85-0.95): Smooth, minimal absorption, excellent collection
- Concrete/RCC (0.80-0.90): Good collection, some absorption when new
- Tile Roof (0.70-0.80): Porous surface, moderate absorption
- Asphalt Shingles (0.75-0.85): North American standard
System Components
- Catchment: Roof surface that collects rainwater
- Gutters: Channels that transport water from roof edges
- First Flush: Device to discard initial dirty water
- Filters: Remove leaves, debris, and contaminants
- Storage Tank: Underground or overhead water storage
- Distribution: Piping system to use points
System Design Tips
Tank Sizing
Size tank for 3-6 months household needs or 80% annual collection (whichever is lower). This balances investment with dry season supply.
First Flush Diverter
Install a first flush system to discard the first 20-30 liters which contain roof dust and bird droppings.
Multiple Filtration
Use 2-3 stage filtration: coarse mesh (remove leaves), sand filter (particles), and charcoal filter (impurities).
Overflow Management
Direct overflow to garden, recharge pit, or stormwater drain. Never let overflow damage building foundation.
Groundwater Recharge
If tank is full, direct excess to recharge pits or bore wells to replenish groundwater table.
Regular Maintenance
Clean gutters monthly during monsoon, clean tank annually, check filters weekly, and inspect for cracks regularly.
Frequently Asked Questions
Is rainwater harvesting mandatory in India?
Many states mandate RWH for buildings above certain sizes. Tamil Nadu requires it for all buildings, Delhi for plots over 100 m², Mumbai for buildings over 300 m². Check local municipal regulations for specific requirements.
Can I drink harvested rainwater?
Rainwater needs treatment before drinking. Use for gardening, washing, and toilet flushing directly. For drinking, install proper filtration (sand, UV, RO) and regularly test water quality. First flush and clean storage are essential.
What's the cost of installing a rainwater harvesting system?
Basic system (1000L tank, filters, piping): ₹15,000-30,000. Mid-range (5000L underground tank): ₹50,000-1,00,000. Premium (10,000L with advanced filtration): ₹1,50,000-3,00,000. ROI typically 3-5 years through water bill savings.
How long can I store rainwater?
In covered, sealed tanks: 6-12 months if properly filtered. Underground tanks stay cooler and water lasts longer. Add chlorine tablets every 2-3 months to prevent algae growth. Check water quality before use after long storage.
What if my area has low rainfall?
Even 400-600mm rainfall can harvest significant water. Focus on non-potable uses (gardening, car wash, toilet). Use larger roof area, increase runoff coefficient with proper surface. Consider groundwater recharge if storage is impractical.
Do I need permission to install rainwater harvesting?
Generally no permission needed for private residential RWH. However, notify municipal body if connecting to existing water supply or doing major structural work. Some areas offer subsidies - check with local water department.
Can apartment buildings implement rainwater harvesting?
Yes! Apartment RWH is highly effective due to large roof area. Society can install centralized system with underground tank. Cost divided among residents. Typical 20-unit building can harvest 200,000-500,000 liters annually.
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