Rainwater Harvesting Calculator
1 · Catchment Areas
Storage requirements based on BWSSB bye-laws for Bengaluru
Clean Roof Area
Coeff: 0.9 · BWSSB: 60 L/sqm storage required
sq.m
Paved Area / Driveway
Coeff: 0.7 · BWSSB: 30 L/sqm storage required
sq.m
Landscape / Garden
Coeff: 0.3 · Not regulated under BWSSB byelaws
sq.m
2 · Underground Storage Available
Raw / Fire UG Sump
Usable for receiving harvested rainwater (raw)
KL
Treated Water UG Sump
If RWH can be routed post-WTP
KL
3 · Daily Water Supply Sources
Current Supply (per day, per source)
Used to calculate what fraction of total supply RWH can offset annually
BWSSB / Municipal
KL / day
Borewell / Groundwater
KL / day
Private Tanker / Other
KL / day
Runoff Summary
Annual Runoff Potential
—
KL / year
BWSSB Min. Storage
—
KL capacity
Total Catchment
—
sq.m
Runoff by Surface
| Surface | Area (sqm) | Coeff | Annual (KL) | 30mm (KL) | 60mm (KL) | 100mm (KL) |
|---|
Single Event Runoff
Moderate Rain
30
mm per event
—
KL generated
Heavy Rain
60
mm per event
—
KL generated
Very Heavy Rain
100
mm per event
—
KL generated
BWSSB Storage Compliance
Required Storage vs Available UG Capacity
Notes
Default annual rainfall 974.5 mm (~60 rainy days) reflects Bengaluru averages — adjust the value above for other cities. Roof coefficient 0.90 applies to all clean roof surfaces; contaminated areas (cooling towers, dumping zones) must be excluded and diverted to drain. BWSSB mandates minimum 60 L/sqm storage for rooftop catchments and 30 L/sqm for paved areas. For sites with deep basements, surface runoff recharge via injection wells is preferred over shallow pit recharge to avoid worsening seepage. Water quality testing required before integrating harvested rainwater into potable supply.
Default annual rainfall 974.5 mm (~60 rainy days) reflects Bengaluru averages — adjust the value above for other cities. Roof coefficient 0.90 applies to all clean roof surfaces; contaminated areas (cooling towers, dumping zones) must be excluded and diverted to drain. BWSSB mandates minimum 60 L/sqm storage for rooftop catchments and 30 L/sqm for paved areas. For sites with deep basements, surface runoff recharge via injection wells is preferred over shallow pit recharge to avoid worsening seepage. Water quality testing required before integrating harvested rainwater into potable supply.
From potential to implementation
These figures are indicative estimates based on catchment area and rainfall data. Whether and how much of this potential can actually be realised depends on several site-specific factors:
- Feasibility and site conditions — topography, available space, existing drainage and plumbing layout, structural constraints, and current water quality.
- Hydrogeology — depth to water table, aquifer type, soil permeability, and proximity to drainage channels or water bodies determine what recharge options are viable.
- Solution design — storage vs. recharge vs. direct use, pre-treatment requirements, integration with existing supply systems, and phasing all need site-specific design.