RainbowHome guide · Rainwater harvesting · Karnataka & Tamil Nadu
How much rainwater can you harvest from your roof? (calculation guide)
There is one simple formula behind every rainwater-harvesting estimate. This page shows you exactly how to work out your roof's yield in litres a year, with worked examples for Bengaluru and Chennai, a runoff table by roof type, and how to turn the number into a tank size. It is a plain-English guide — no quote form.
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The formula
Harvest (litres/year) = Roof area (m²) × Annual rainfall (mm) × Runoff coefficient
For an RCC roof, the runoff coefficient is about 0.8. The units work out neatly: 1 mm of rain on 1 m² of roof is exactly 1 litre, so multiplying millimetres by square metres already gives litres — the coefficient just discounts what you lose.
- Roof area (m²) — the flat plan footprint of the catchment seen from above, not the sloped surface area. This is what actually intercepts rain.
- Annual rainfall (mm) — the long-period average for your location. One millimetre means one litre fell on every square metre over the year.
- Runoff coefficient — the fraction of that rain that reaches your tank after evaporation, surface wetting, splash and first-flush diversion. Smooth surfaces keep more; rough or porous ones keep less.
Step by step
- Measure your roof plan area. Length × breadth of the flat footprint, in metres. If you only have square feet, divide by 10.76 to get square metres (1,000 sq ft ≈ 93 m²).
- Get your local annual rainfall.Use the long-period average for your city — Bengaluru is roughly 900–1,000 mm and Chennai roughly 1,300–1,400 mm (IMD normals; see sources below). Use your own town's figure where you can.
- Pick a runoff coefficient by roof type from the table below — about 0.8 for RCC, higher for metal, lower for tile.
- Multiply the three numbers. Area × rainfall × coefficient = litres you could harvest in an average year.
- Convert to tanker loads and storage size. Divide by a typical tanker capacity to picture the volume, then size storage for a few dry days and route the rest to recharge.
Worked examples
Both examples use a 100 m² RCC roof (coefficient 0.8) and the average annual rainfall for each city. A typical Bengaluru water tanker is around 6,000 litres — useful for picturing the volume.
| City | Rainfall (avg) | Calculation | Harvest / year | ~6,000 L tankers |
|---|---|---|---|---|
| Bengaluru | ~970 mm | 100 × 970 × 0.8 | ~78,000 L | ~13 |
| Chennai | ~1,400 mm | 100 × 1400 × 0.8 | ~1,12,000 L | ~18–19 |
Figures rounded. Rainfall varies year to year; a dry year in Bengaluru might yield closer to 60,000 L and a strong northeast monsoon in Chennai well over 1,20,000 L. Size on the average, plan for the swing.
Runoff coefficient by roof type
| Surface | Typical coefficient | Notes |
|---|---|---|
| RCC / concrete roof | 0.75–0.85 | The common default (~0.8) for Indian homes. |
| Metal / GI sheet | 0.85–0.90 | Smooth and sloped — sheds water fast, low absorption. |
| Tiled / Mangalore-tile roof | 0.60–0.75 | Joints and porosity absorb and leak more. |
| Unpaved ground / lawn | 0.10–0.30 | Mostly infiltrates on the spot; poor as a catchment. |
Ranges are standard planning values consistent with CGWB and IS rainwater-harvesting guidance. Real performance depends on roof condition, slope, and whether you run a first-flush diverter.
Storage vs recharge — what to do with the volume
Once you know the litres, you have two ways to use them, and most homes do both:
- Storage.Collect rain in a tank to use directly for flushing, washing and gardening. You size a tank to bridge dry spells between rain events — typically a few days' to a few weeks' demand, not a whole year's harvest.
- Recharge.Send the surplus (and the overflow from your tank) into a recharge pit or well so it refills the groundwater your borewell draws from. In Chennai's short-and-intense monsoon, recharge is often the larger share.
For the practical build — first-flush diverter, filter, tank and pit sizing — see our step-by-step how to set up rainwater harvesting at home guide, and what it should cost in our RWH cost benchmark. If your building falls under a local rule, check the Karnataka or Tamil Nadu rainwater-harvesting mandate.
Skip the arithmetic
Enter your roof area and city and our calculator does the formula for you, then suggests a storage-and-recharge split and a cost band. It is neutral and free — see how we rank and our methodology.
Frequently asked questions
How do you calculate rainwater harvesting potential from a roof?
Multiply three numbers: roof plan area in square metres × annual rainfall in millimetres × a runoff coefficient (about 0.8 for an RCC roof). The result is litres you could collect in an average year. Example: a 100 m² RCC roof where 970 mm of rain falls yields roughly 100 × 970 × 0.8 ≈ 77,600 litres a year. Rainfall varies year to year, so treat any single figure as a typical estimate, not a guarantee.
What is the runoff coefficient for an RCC roof?
A smooth reinforced-cement-concrete (RCC) roof is usually taken at about 0.75–0.85 (commonly 0.8), meaning roughly 75–85% of the rain that lands on it can be captured after evaporation, wetting and first-flush losses. Sloping GI or metal-sheet roofs run higher (about 0.85–0.9), tiled roofs lower (about 0.6–0.75), and unpaved ground much lower (about 0.1–0.3). These are standard planning ranges from CGWB and IS guidance; your site conditions can shift the real figure.
How many litres of rainwater can a 1000 sq ft roof collect?
1,000 sq ft is about 93 m². On an RCC roof (coefficient ~0.8) in a place that gets around 970 mm a year (typical of Bengaluru), that is roughly 93 × 970 × 0.8 ≈ 72,000 litres in an average year. In a wetter city like Chennai (around 1,400 mm), the same roof could yield roughly 1,04,000 litres. Actual yield depends on the year's rainfall and how much you can store or recharge.
How much rain does Bengaluru get in a year?
Bengaluru's long-period average annual rainfall is roughly 900–1,000 mm (IMD normals put it near 970 mm), spread across the southwest and northeast monsoons. Individual years swing well above and below this, so size your system on the average but expect variation.
How much rain does Chennai get in a year?
Chennai's long-period average is roughly 1,300–1,400 mm a year, with the bulk arriving in the northeast (retreating) monsoon from October to December. Because so much falls in a short window, storage and recharge capacity matter more than in evenly-rained cities.
How big a tank do I need to store my roof's rainwater?
You rarely size a tank to hold a whole year's harvest — that would be enormous. Instead, size it to bridge dry spells between rain events, then send the overflow to a recharge pit. A common rule of thumb for Indian homes is a few days' to a few weeks' demand: many homes install 5,000–10,000 litres of storage and recharge the rest. Our water calculator suggests a storage and recharge split for your roof area and city.
Last verified: July 2026
Sources: rainfall normals from the India Meteorological Department (IMD), mausam.imd.gov.in; runoff-coefficient and recharge guidance from the Central Ground Water Board (CGWB), cgwb.gov.in. City rainfall figures are long-period averages and vary year to year; runoff coefficients are typical planning ranges, not site-specific measurements. For local rainwater-harvesting rules, see bwssb.karnataka.gov.in (Bengaluru) and chennaimetrowater.tn.gov.in (Chennai).