📖 How To Use
How to Use the Rainwater Harvesting Calculator
Calculate your roof's annual rainwater collection potential in under a minute:
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Measure your roof catchment area
Enter the horizontal length and width of your roof footprint — use the plan (top-down) dimensions, not the sloped surface. For example, a house 12 m long × 8 m wide has a 96 m² catchment area regardless of roof pitch.
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Enter your annual rainfall
Look up your location's average annual precipitation from a local weather service or the World Bank Climate Data portal. Enter it in mm (millimetres) or inches. Global averages range from under 100 mm (arid) to over 3,000 mm (tropical).
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Select your roof type
The dropdown sets the runoff coefficient — the fraction of rain that is actually collectible after accounting for evaporation, splash loss, and absorption. Metal roofs retain the most (0.90); green roofs absorb the most (0.60).
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Enter your daily water usage (optional)
Inputting your household's daily water consumption lets the calculator show how many days per year your harvested rainwater covers your demand and what percentage of your annual usage it can supply.
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Read your results
The primary result shows annual harvestable litres. The secondary grid shows monthly/daily breakdown, recommended tank size (based on 1–2 months' peak collection), supply ratio, and more.
Tip: For multi-section roofs, calculate each section separately and add the yields together. Only connect gutters that drain toward your collection point — a section draining to the street adds nothing to your harvest.
📐 The Formula
Rainwater Harvesting Formula Explained
The standard rainwater harvesting yield formula used worldwide by engineers and planners:
Annual Yield (L) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient (Cr)
Where:
Roof Area = Length (m) × Width (m) [horizontal footprint]
Annual Rainfall = mm/year from local weather data
Runoff Coefficient (Cr) = 0.3 – 0.95 depending on roof surface
Note: 1 mm of rain on 1 m² of roof = 1 litre of water (before losses)
This formula works because 1 mm of rainfall depth across 1 square metre equals exactly 1 litre. Multiplying by the runoff coefficient accounts for real-world collection losses.
Runoff Coefficients by Roof Type
| Roof Material | Runoff Coefficient (Cr) | Notes |
|---|
| Metal / Tin / Zincalume | 0.90 – 0.95 | Best for harvesting; minimal absorption |
| Concrete / Flat Tile | 0.80 – 0.90 | Common residential; good efficiency |
| Asphalt Shingle | 0.70 – 0.80 | Some absorption; first-flush critical |
| Clay / Terracotta Tile | 0.65 – 0.75 | Porous surface absorbs initial rain |
| Green / Living Roof | 0.50 – 0.65 | High absorption; significant retention |
| Gravel / Rough Membrane | 0.25 – 0.40 | Significant losses; not recommended |
Recommended Tank Size
This calculator recommends a tank sized to capture approximately 6 weeks of average rainfall collection — balancing cost, space, and yield. The formula used is:
Recommended Tank (L) = (Annual Yield ÷ 12) × 1.5
[i.e., 1.5 × average monthly collection]
For regions with distinct wet/dry seasons, size the tank to capture an entire wet-season month's peak rainfall rather than the annual average.
📏 Reference Data
Rainwater Yield by City — Example Calculations
Using a 100 m² roof with a 0.85 runoff coefficient (concrete tile):
| City | Annual Rainfall (mm) | Yield (100 m², Cr=0.85) | Yield (US gal) |
|---|
| London, UK | 601 | 51,085 L | 13,493 gal |
| Sydney, AU | 1,213 | 103,105 L | 27,233 gal |
| Mumbai, IN | 2,167 | 184,195 L | 48,642 gal |
| Los Angeles, US | 381 | 32,385 L | 8,550 gal |
| Cape Town, ZA | 515 | 43,775 L | 11,561 gal |
| Karachi, PK | 196 | 16,660 L | 4,401 gal |
| Singapore | 2,340 | 198,900 L | 52,527 gal |
| Berlin, DE | 570 | 48,450 L | 12,795 gal |
Note: Annual rainfall figures are long-term averages. Actual yields vary year-to-year. Use local meteorological data for project-level planning.
❓ FAQ
Frequently Asked Questions
How do I calculate how much rainwater I can collect from my roof?
Multiply your roof's catchment area (m²) by your annual rainfall (mm) and by your roof's runoff coefficient. For example, a 100 m² concrete tile roof in a city with 800 mm annual rainfall: 100 × 800 × 0.85 = 68,000 litres per year. This calculator does this math instantly.
What is a runoff coefficient and why does it matter?
The runoff coefficient (Cr) represents the proportion of rainfall that actually reaches your storage tank after losses from evaporation, splash, initial surface wetting, and first-flush diversion. A metal roof has Cr ≈ 0.90, meaning 90% of rainfall is collectible. An asphalt shingle roof has Cr ≈ 0.75. Using the wrong coefficient can over- or under-size your system by 15–30%.
What size rainwater tank do I need?
Tank size depends on your daily demand and the gap between rainfall events. A common rule of thumb is to size the tank to hold 4–8 weeks of average collection, or 1–2 months of your household's daily water usage, whichever is larger. This calculator recommends 1.5× your average monthly yield as a starting point. Use our First Flush Diverter Size Calculator for additional guidance on pre-tank filtration volumes.
How much of my roof area should I use for the calculation?
Use the horizontal footprint (plan area) of your roof, not the sloped surface area. A 10 m × 8 m house has 80 m² of catchment regardless of pitch angle — because rainfall is measured vertically. Only include roof sections whose gutters drain to your collection tank. Exclude areas that drain to the street or neighbouring properties.
Is harvested rainwater safe to drink?
Roof-collected rainwater is not automatically safe for drinking. It can pick up dust, bird droppings, atmospheric pollutants, and moss from the roof surface. For potable use, it requires first-flush diversion, filtration (sediment + carbon), and disinfection (UV or chlorination). For non-potable uses — toilet flushing, laundry, irrigation — basic sediment filtration is usually sufficient. Always comply with your local health authority's guidelines.
What is a first flush diverter and do I need one?
A first flush diverter automatically discards the first few litres of rain from each storm event — the most contaminated water that washes accumulated dust, bird droppings, and debris off the roof. Most engineers recommend diverting the first 1–2 mm of rainfall per event (1–2 litres per m² of catchment). This device significantly improves water quality and is considered best practice for all potable and many non-potable systems. Use our First Flush Diverter Size Calculator to size yours.
How does roof pitch affect rainwater collection?
Roof pitch does not affect the volume of water you can collect — only the horizontal footprint matters, because rainfall is measured as a vertical depth. A steep pitch actually helps by improving drainage speed and reducing standing water, which reduces evaporative loss and algae growth. Very shallow pitches (under 5°) can pool water and accumulate sediment, slightly reducing effective yield.
Can a rainwater harvesting calculator tell me my ROI?
This calculator gives you the yield data needed to calculate ROI. Take your annual yield in litres, multiply by your local water rate (cost per litre or per m³), and compare to your system's installation cost. Typical residential systems pay back in 3–8 years depending on rainfall, local water tariffs, and system size. Use our Rainwater Harvesting ROI Calculator for a full financial analysis.