Interactive Calculator · 2026
Rainwater + STP Integration Calculator
Combine rooftop rainwater harvesting with STP treated-water reuse to see the total non-potable water your building can supply — in KL per year. Enter your catchment, rainfall and STP reuse to get the one number a dual-water strategy starts from.
Rainwater vs STP treated-water — share of non-potable supply
Your site & systems
Total roof or paved area draining to storage.
Local mean annual precipitation for your city.
Fraction of rainfall actually captured — ≈0.8 for RCC roofs, lower for paved / green surfaces.
Treated water reused each day (KLD) for flushing, landscape and cooling — from your STP.
Where your non-potable water comes from
Together the two sources supply ≈59,120 KL/yr — STP reuse contributes 98.8% of it:
Rainwater is seasonal and free but capped by roof area and rainfall; STP reuse is steady year-round. Sizing storage and demand against both is what lets a building cut its freshwater purchase to a minimum.
Use rainwater first during the monsoon, then lean on treated STP water through the dry months so no reusable litre is wasted.
How this is calculated
- Rainwater = area × (rainfall ÷ 1000) × runoff = 1,000 m² × 0.9 m × 0.80 = 720 KL/yr.
- STP reuse = reuse × 365 days = 160 KLD × 365 = 58,400 KL/yr.
- Total non-potable = rainwater + STP reuse = 720 + 58,400 = 59,120 KL/yr, of which STP is 98.8%.
Indicative planning figures. Actual yield depends on rainfall pattern, first-flush losses, storage volume and reuse demand — confirm with a qualified consultant before design.
Related Guides — Deep-dive reading
Rooftop Water Recycling Integration in Homes: One Non-Potable System from Rain and Greywater
How to combine rooftop rainwater harvesting with greywater and STP reuse into a single non-potable water system — the storage, dual plumbing, controls and seasonal logic that make two sources behave like one reliable supply.
Sewage Treatment PlantsCircular Water Management for Buildings & Cities: The Closed-Loop Guide
How to stop treating water as a one-way flow and design a closed loop — treat, reuse, recharge, repeat — with a sewage treatment plant at its core, for Indian buildings, campuses and cities.
Sewage Treatment PlantsWater-Positive Buildings: The Role of STPs
What "water positive" actually means, how treated STP water, rainwater harvesting and groundwater recharge combine to get a building there, and how to measure the balance credibly in the Indian context.
Sewage Treatment PlantsRelated Tools — You may also find these useful
Green Building Water Score Calculator
Score a building's water efficiency 0–100 from treated-water reuse, rainwater harvesting, low-flow fixtures and metering, with an IGBC/GRIHA-style rating and your biggest improvement lever.
SustainabilitySTP vs Septic Tank Decision Tool
Answer five yes/no questions about your site and get a clear STP-or-septic-tank recommendation, with a confidence score and the reasons driving it.
Decision ToolTreated Water Reuse Planner
Allocate a building's treated water across flushing, landscape, cooling and washing, and see the remaining volume for groundwater recharge — all in KLD.
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