📖 How To Use
How to Use This Rainwater Savings Calculator
It takes less than a minute to see your full annual rainwater harvesting potential:
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Enter your roof or catchment area
Measure the horizontal footprint of your roof — not the sloped surface. A 10 m × 8 m house has an 80 m² catchment area regardless of roof pitch. Choose m² or ft² to match your tape measure.
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Enter your local annual rainfall
Find your city's average annual rainfall from a weather authority or BOM/NOAA website. Enter in mm, cm, or inches — whatever your source uses. Global averages range from under 200 mm (arid desert) to over 3,000 mm (tropical rainforest).
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Set the runoff coefficient
This accounts for water lost to evaporation, absorption, and first-flush diversion. Use 0.85–0.90 for metal/tile roofs, 0.75–0.85 for asphalt shingles, and 0.60–0.70 for gravel or green roofs. If unsure, 0.80 is a safe default.
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Enter your water tariff
Input your utility's price per kilolitre (kL = 1,000 litres). If your bill shows price per cubic metre (m³), it's the same number. If it shows per US gallon, multiply by 264.
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Enter system cost (optional)
If you know your installation cost for tanks, pumps, and guttering, enter it here to get a payback period estimate. Leave blank or zero to skip.
Tip: Only count roof area that drains into your collection system. If one side of your roof drains to the street, use only the area that feeds your gutters and downpipes.
📐 The Formula
Rainwater Savings Formula Explained
The calculation uses two chained equations — one for collection volume, one for financial savings:
Annual Collection (L) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient
Annual Savings = (Collection ÷ 1,000) × Water Rate per kL
Payback Period (yrs) = System Cost ÷ Annual Savings
The roof area × rainfall gives the theoretical maximum rainfall volume in litres (1 mm of rain on 1 m² = exactly 1 litre). The runoff coefficient scales this down to account for real-world losses.
Runoff Coefficient Reference
| Roof / Surface Type | Runoff Coefficient | Notes |
|---|
| Colorbond / Metal | 0.90–0.95 | Very high efficiency, minimal absorption |
| Clay / Concrete Tile | 0.85–0.90 | Good efficiency when wet |
| Asphalt Shingles | 0.75–0.85 | Some absorption, especially when dry |
| Slate | 0.85–0.90 | Similar to tile |
| Green / Living Roof | 0.30–0.60 | High absorption by growing media |
| Gravel Roof | 0.60–0.70 | Significant retention in aggregate |
| Paved Courtyard | 0.70–0.85 | Depends on surface smoothness |
Annual Rainfall by Region
| City | Annual Rainfall (mm) | Classification |
|---|
| London, UK | 601 | Temperate maritime |
| Sydney, Australia | 1,213 | Humid subtropical |
| Mumbai, India | 2,167 | Tropical monsoon |
| New York, USA | 1,174 | Humid continental |
| Dubai, UAE | 94 | Hot desert |
| Karachi, Pakistan | 196 | Hot arid |
| Cape Town, S. Africa | 515 | Mediterranean |
| São Paulo, Brazil | 1,450 | Humid subtropical |
❓ FAQ
Frequently Asked Questions
How do I calculate rainwater savings from my roof?
Multiply your roof's catchment area (m²) by your local annual rainfall (mm) by the runoff coefficient. This gives litres collected per year. Divide by 1,000 to get kilolitres, then multiply by your water tariff ($/kL) to get your annual dollar savings. This calculator does all of that automatically.
What is a runoff coefficient and what should I use?
The runoff coefficient (0 to 1.0) represents what fraction of rainfall actually reaches your tank after accounting for evaporation, absorption by the roof material, and first-flush diversion losses. Use 0.85–0.90 for metal roofs, 0.80–0.85 for tiles, 0.75–0.80 for asphalt shingles. When in doubt, 0.80 is a conservative and reasonable default for most residential roofs.
How much water can a typical house collect per year?
A typical 150 m² home in a city with 700 mm annual rainfall and a 0.85 runoff coefficient collects about 89,250 litres (89 kL) per year. That's roughly 245 litres per day — enough to cover most garden irrigation, toilet flushing, and laundry needs. In high-rainfall cities like Mumbai or Sydney, the same roof could collect 200,000+ litres annually.
How long does a rainwater harvesting system take to pay back?
Payback periods typically range from 3 to 12 years depending on system cost, local rainfall, roof area, and water tariff. A basic 5,000-litre tank system costing $2,000 with $400/year in savings pays back in 5 years. After payback, all savings are pure gain — a system can last 20–30 years. Enter your system cost into the calculator to see your personal payback period.
Does roof pitch affect how much rainwater I can collect?
No — roof pitch does not affect collection volume. You should use the horizontal footprint area (plan area), not the actual sloped roof surface. The rainfall measurement already represents vertical depth of water falling, so horizontal area × vertical rainfall gives the correct volume regardless of pitch angle.
What is the CO₂ offset calculation based on?
The CO₂ estimate uses an average energy intensity for mains water treatment and distribution of approximately 0.35 kWh per kilolitre, combined with a grid carbon factor of around 0.5 kg CO₂/kWh, yielding roughly 0.19 kg CO₂ offset per kilolitre of rainwater used. This varies significantly by region — areas with pumped groundwater or desalination have much higher carbon intensity. The figure is indicative only.
Can I use this calculator for rainwater harvesting ROI?
Yes. Enter your total system installation cost (tanks, pumps, filters, guttering) in the System Cost field. The calculator will show your payback period in years and 10-year cumulative savings. For a more detailed return-on-investment analysis including maintenance costs, see our Rainwater Harvesting ROI Calculator.
What size tank do I need to store the rainwater I collect?
Tank sizing depends on collection rate, consumption rate, and the dry season gap between rainfalls. A useful rule of thumb: size your tank to hold at least 1–2 months of your average monthly collection. Use our Rainwater Harvesting Calculator for detailed tank sizing based on seasonal rainfall patterns.