Utility
Rainwater Tank Sizer
How big should your rainwater harvesting tank be? Enter roof area, roof type, household size, and city — get annual harvest, recommended tank size in litres, water-bill savings, and payback.
Roof
1076 sq.ft · typical urban home 80–200 m² (860–2150 sq.ft)
Most common Indian urban rooftop; high yield if parapet drains are clear.
Household
BIS / CPHEEO norm: 135 L/person/day for fully-plumbed urban homes. Daily household demand: 540 L.
Days of household demand the tank must carry between refills. 15–30 is typical.
City & economics
Jun–Oct (SW monsoon + NE retreat). Dry spell: ~120 days/year.
Default: Bengaluru (BWSSB, above 25 kL slab). Override if you’re in a higher slab or a gated community with internal tariff.
Tank + first-flush + filter + plumbing. Typical range ₹40,000–1,20,000.
Recommended tank capacity
Annual harvest
63.85 kL
Bengaluru roof
Peak month
12.77 kL
20% of annual
Demand coverage
32%
of annual water needs
Demand-driven size (20 days)
10.80 kL
Supply-driven size (peak month)
12.77 kL
Economics
Annual bill savings
₹2,873
@ ₹45/kL
System cost
₹60,000
tank + fittings
Payback
20.9 years
at today's tariff
Tariffs rise ~7–10%/year; actual payback is shorter. First-flush per event: 200 L (2 mm/m² CGWB spec).
Likely mandatory in your city
BWSSB: mandatory for plots ≥ 40×60 ft (~223 m²) since 2009; ≥ 30×40 ft (~112 m²) from 2021.
Check with your architect or local authority before plan sanction — most cities enforce RWH at building-plan approval.
How this works
- Annual harvest = roof area (m²) × annual rainfall (m) × runoff × filter efficiency, minus first-flush losses. Rainfall figures are IMD long-term averages for each city.
- Runoff coefficient — fraction of incident rainfall that actually reaches the tank. Smooth non-absorbent surfaces (metal sheet, clean RCC) run off ~85–90%; porous finishes (green roof, gravel) run off 40–70%. IS 15797:2008 Table 2.
- First flush — CGWB recommends diverting 2 mm/m² of roof area per event before water enters the tank. This removes bird droppings, dust, and leaf litter that would otherwise foul the storage within weeks.
- Recommended tank size = the smaller of (peak-month harvest) and (storage-days × daily demand), with a floor at the longest dry spell. Bigger than peak-month is wasted capacity; smaller than a dry spell leaves you exposed.
- What this doesn’t include: groundwater recharge volume (additional 30–60% of roof runoff can be directed to a soak pit), partial-rainy-season reservoir effects, and quality differences by roof age. For exact compliance drawings, use your architect.
Deep-dive: Rainwater Harvesting for Indian Urban Homes — Complete Guide
Related: Biophilic Score · Plot Evaluation · Cross-Ventilation Analyzer
Indicative estimate. Local site conditions — roof slope, gutter efficiency, filter maintenance, and microclimate — can vary actual yield by ±20%. First year is usually 10–15% lower while the roof & filters “season”.
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Reference Chart