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Rainwater Harvesting System Calculator

Size rainwater harvesting system - collection tank, recharge pit and self-sufficiency estimate. NBC Part 9 / IS 15797 compliant. Free tool.

📐 Standard: NBC Part 9 / IS 15797
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

Rainwater harvesting (RWH) is now mandatory in many Indian states for buildings above a specified plot area. It reduces demand on piped municipal supply, recharges groundwater, and earns IGBC/GRIHA green building credits. Many municipalities offer property tax rebates of 10–30% for buildings with functional RWH systems. IS 15797 and NBC Part 9 provide the design framework.

States with mandatory RWH: Karnataka (plots > 30×40ft in BBMP area), Tamil Nadu (all new buildings in Chennai), Delhi (plots > 500 m²), Maharashtra (plots > 1000 m²), AP & Telangana (all new buildings). The IS 15797 standard covers system components, filter design, and water quality. First flush devices - which divert the first 2–3mm of rainfall that washes accumulated dust and pollutants off the roof - are essential for water quality.

📐 Rainwater Harvesting Calculation (NBC Part 9)

NBC Part 9 / IS 15797

Annual Harvest Volume:
  V_annual = A × (R − FF × 12) × Rc / 1000

Where:
  A    = Catchment (roof) area (m²)
  R    = Annual rainfall (mm/year)
  FF   = First flush depth (mm/rain event, typically 2.5mm)
  12   = average rain events per year
  Rc   = Runoff coefficient (0.9 for RCC roof)

Storage Tank:
  V_tank = Q_daily_demand × storage days (15 days min)

Recharge Pit:
  A_pit = V_annual × recharge fraction / soil absorption rate
  Depth: 3–6m, lined with gravel filter

❓ Frequently Asked Questions

Is rainwater harvesting mandatory in India? +
Yes, in several states and cities. Karnataka mandates RWH for all new buildings on plots 30×40 ft and above in BBMP limits. Tamil Nadu mandates it for all buildings in Chennai. Delhi: plots above 500 m². Maharashtra: plots above 1000 m². Many municipalities link building completion certificates and occupancy certificates to the presence of functional RWH systems. Check local bylaws.
What is the runoff coefficient for different roof types? +
RCC / concrete roof: 0.85–0.90. Tiled roof (clay): 0.80–0.85. Asbestos sheet (AC sheet): 0.80. Metal sheet roof: 0.90. Green roof / terrace garden: 0.30–0.50. The runoff coefficient accounts for losses to evaporation and surface absorption. Concrete roofs give the highest yield; green roofs significantly reduce runoff.
How much roof area do I need to supply 500 L/day from rainwater? +
For Bangalore (800mm/yr rainfall, 0.85 Rc, 20% first flush loss): Annual yield per m² ≈ (800 × 0.85 - 30) / 1000 = 0.65 m³/m²/year = 1.78 L/m²/day. For 500 L/day: required area = 500/1.78 ≈ 280 m² of roof. In higher rainfall areas like Mumbai (2000mm), the same 500 L/day needs only about 100 m² of catchment.
What is a first flush diverter and why is it needed? +
A first flush diverter automatically discards the first 2–3mm of rainfall from each event (equivalent to 2–3 litres per m² of roof). This water carries dust, bird droppings, atmospheric pollutants, and leaf debris. Discarding it significantly improves the quality of the harvested water for subsequent rain. IS 15797 requires first flush diverters in all RWH systems intended for domestic use.
Can harvested rainwater be used for drinking? +
Not directly without treatment. Harvested rainwater from rooftops is suitable for toilet flushing, garden irrigation, cooling tower makeup, and car washing - non-potable uses. For potable use, it requires filtration and disinfection (sand filter + activated carbon + UV/chlorination) to meet IS 10500 drinking water standards. In practice, most Indian RWH systems use harvested water for non-potable purposes.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

🌧️ Rainwater Harvesting Calculator
Reference: NBC Part 9 / IS 15797