Design a system that collects rain off a roof, stores it and uses it for toilet flushing and garden irrigation. Adjust the catchment area, annual rainfall and tank volume to see — in real time — how much rainwater you can collect each year and how much of your demand it covers.
Parameters
Roof catchment area
m²
Horizontal projected area of the roof that catches rain
Annual rainfall
mm
Total yearly precipitation for the location
Runoff coefficient
Factor for splash, evaporation and surface-wetting losses
Filter / first-flush efficiency
Fraction remaining after first-flush diversion and filtering
Daily demand
L/day
Daily water volume you want rainwater to cover
Storage tank volume
L
Capacity of the buffer tank that stores irregular rain
Results
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Annual harvest (m³)
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Annual demand (m³)
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Supply coverage (%)
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Tank-only supply (days)
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Water saved/year (m³)
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System rating
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Rainwater system schematic — collection animation
Rain on the roof runs through a gutter, a first-flush diverter and a filter into the storage tank, and a tap draws it for use. The tank's water level reflects the supply coverage.
Annual harvestable volume V_harvest [L]. Since 1 mm of rain on 1 m² of roof gives exactly 1 litre, the yield is area × rainfall directly. C_runoff: runoff coefficient, η_filter: filter / first-flush efficiency.
Days the full tank alone could supply. V_tank: tank volume [L]. The storage tank buffers the irregular supply against the steady, daily demand.
What is the Rainwater Harvesting Simulator?
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"Rainwater harvesting" means collecting the rain off a roof and using it, right? But does rain really add up to much?
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It actually adds up to a lot. There is an easy relationship to remember: 1 mm of rain on 1 m² of roof gives exactly 1 litre. So a 120 m² roof with 1600 mm of annual rain sees 120 × 1600 = 192,000 litres — 192 tonnes of water — just running off it. Rainwater harvesting catches that instead of letting it go down the storm drain. It is one of the oldest pieces of water engineering humans have ever practised.
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Can you use all 192 tonnes? That's amazing — but when I lower the "runoff coefficient" or "filter efficiency" on the left, the harvest drops.
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Good catch — you can't get all of it. Some of the rain splashes off, some evaporates in sunny spells, and some is used up just wetting a dry roof surface. The runoff coefficient captures that — about 0.8 to 0.9 for tile or metal roofs. The filter efficiency then accounts for the "first-flush" diverter, which deliberately throws away the dirty first few litres of each storm (dust, bird droppings, leaves), plus filter rejection. With the default values the harvest works out to about 146.9 m³ — still plenty.
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The harvest is 146.9 m³ and the annual demand is 146.0 m³ — almost the same. So can the tank be small?
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That's where design gets hard. Even when the annual totals balance, rain arrives in irregular bursts, while you use water at a steady daily rate. After a typhoon dumps a lot at once, it can stay bone-dry for two weeks. You have to bridge that dry spell with water stored in the tank. So the tank is a buffer that bridges the timing mismatch between supply and demand — and sizing it is the central design decision of a rainwater-harvesting system.
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I see. So bigger is always better for the tank?
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No — too big is a problem too. Tanks cost money and take up space. A useful guide is "days of autonomy = tank volume ÷ daily demand": a 5000 L tank at 400 L/day gives 12.5 days. As long as that covers your region's longest typical dry spell — a dry summer after the rainy season, say — it is enough; beyond that is wasted money. Move the rainfall slider on the chart below to see how coverage changes. In dry regions, a bigger tank cannot help because the rainfall itself is the ceiling.
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You can't drink the collected rainwater, can you? What's it best used for?
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Right — untreated, it isn't for drinking. But it is perfectly good for toilet flushing, car washing, cleaning, and especially garden and landscape irrigation — where being untreated is no disadvantage at all. Toilet flushing alone is a large share of household water use, so covering it with rainwater cuts mains supply sharply. And by temporarily holding storm runoff in the tank, the system also eases the load on stormwater drains and softens urban flooding. It does two jobs at once: saving water and protecting against floods.
Frequently Asked Questions
The annual harvestable volume is catchment area × annual rainfall × runoff coefficient × filter efficiency. The key relationship is that 1 mm of rain falling on 1 m² of roof yields exactly 1 litre, so the arithmetic is direct. The runoff coefficient accounts for splash, evaporation and surface wetting losses; the filter efficiency accounts for the first-flush diverter and filter rejection. For a 120 m² roof, 1600 mm of annual rainfall, a runoff coefficient of 0.85 and a filter efficiency of 0.90, you can collect about 146.9 m³ per year.
Rain arrives in irregular bursts, while demand is steady and daily. The tank is the buffer that bridges this gap, and sizing it is the central design decision of a rainwater-harvesting system. A useful guide is days of autonomy = tank volume ÷ daily demand: a 5000 L tank used at 400 L/day gives 12.5 days. Aim to cover the longest typical dry spell in your region (for example a dry summer after the rainy season). An oversized tank wastes money and space without collecting any more water than the rain provides.
Untreated rainwater is not suitable for drinking, but it is perfectly good for non-potable uses: flushing toilets, washing cars, cleaning, and especially garden and landscape irrigation — where the lack of treatment is no disadvantage. Toilet flushing alone accounts for a large share of household water use, so supplying it with rainwater significantly cuts mains consumption. For potable use you must add filtration and disinfection equipment.
A well-sized rainwater-harvesting system delivers two benefits at once. First, it cuts the household's mains-water consumption and bill. Second, by holding back roof runoff during storms in the tank, it eases the load on municipal stormwater drains and reduces local urban flooding. The second benefit — stormwater attenuation — is taken so seriously that many municipalities now offer installation subsidies. The effect of one building is small, but aggregated across a neighbourhood it becomes meaningful flood-resilience infrastructure.
Real-World Applications
Domestic water-saving systems: The most familiar use connects a "rain barrel" or rainwater tank to the roof's downpipe and uses the stored water for garden irrigation, car washing and cooling the pavement in summer. Small tanks of 100-300 L are common, and many municipalities offer installation subsidies. Extending rainwater to toilet flushing needs additional plumbing, but because toilet flushing is a large share of household water use, the saving is substantial.
Large buildings and public facilities: Schools, gymnasiums, office towers and factories have large roof areas and install high-capacity underground or on-site storage tanks. The water serves toilet flushing, cooling-tower make-up and landscape irrigation, and also acts as an emergency water reserve during disasters. Stadiums and airports with roof areas of tens of thousands of square metres can use thousands of tonnes of rainwater per year.
Urban stormwater attenuation (green infrastructure): Recently, rainwater storage aimed at "storm runoff control" rather than water saving has gained importance. Temporarily holding roof rain reduces the peak runoff of short, intense storms to within what the drains can handle, easing urban flash flooding. Combined with rain gardens and permeable paving, it is one element of "green infrastructure" now adopted at the city-planning level.
Water-stressed regions and agriculture: Where rainfall is strongly seasonal or scarce, storing wet-season rain for dry-season domestic and irrigation use is an ancient practice. Beyond roof catchment, large-scale rainwater harvesting combined with ponds and underground storage is, in regions with poor water infrastructure, water security itself. This tool's yield calculation can serve directly as a first-order estimate for such plans.
Common Misconceptions and Pitfalls
The biggest misconception is assuming that "if the annual harvest exceeds the annual demand, rainwater alone is enough". This tool's supply coverage is a comparison of annual totals only; it does not account for the mismatch between when rain falls and when water is used. In reality, if a typhoon dumps a lot at once and then two weeks of clear weather follow, you must survive that period on tank water alone. With a small tank, you can run dry during a drought even though the annual totals balance. Conversely, an extremely large tank cannot collect more than what falls. Even at 100% annual coverage, the actual utilisation rate (the share genuinely supplied by rainwater) typically drops to 70-90% depending on tank size.
Next, the complacency that "the runoff coefficient and filter efficiency are fixed values". The runoff coefficient depends on the roof material — about 0.85 for tile or metal roofs, but as low as 0.5 for a green roof or a rough surface. It also varies with rainfall intensity: a light shower may only wet the roof and produce zero runoff, while a heavy storm raises the coefficient. Filter efficiency likewise varies with the first-flush volume setting, leaf accumulation and maintenance frequency. Estimate these conservatively (on the low side) and treat this tool's result as an ideal upper bound.
Finally, the misconception that "rainwater can be used as-is, untreated". Rainwater carries roof-surface dust, bird droppings and air pollutants, and drinking it untreated is a health risk. Even for non-potable use (toilets, irrigation), water that stagnates in the tank breeds algae and mosquito larvae, so light exclusion, sealing and regular cleaning are essential. Water also goes stale if unused for long periods, so the tank is not "the bigger the better" — it should be sized appropriately for the demand and turnover. For potable conversion, always assume additional treatment such as filtration and UV disinfection.
How to Use
Enter roof area in m² (typical house: 120–200 m²) and select your climate zone to set annual rainfall (temperate: 600–800 mm, tropical: 1200–2000 mm)
Define runoff coefficient (0.75–0.95 for pitched tiles, 0.60–0.80 for flat roofs) and filter efficiency (85–98% removes sediment and debris)
Set tank volume (m³) and monthly demand for toilet flushing (8–15 m³) plus garden irrigation (5–25 m³ depending on season)
Review supply coverage (%), tank autonomy in days, and annual potable water savings in m³
Worked Example
Residential property: 150 m² pitched tile roof in temperate zone (700 mm/year rainfall), runoff coefficient 0.82, filter efficiency 92%, 5 m³ tank. Annual harvest = 150 × 0.700 × 0.82 × 0.92 = 79.4 m³. Monthly demand: 12 m³ (toilets 10 m³ + garden 2 m³). Annual demand = 144 m³. Supply coverage = 79.4/144 = 55%. Tank-only autonomy ≈ 10 days at average flow. Annual mains water saved = 79.4 m³ (≈ 1200 EUR/year in high-cost regions).
Practical Notes
Optimize tank size for local rainfall pattern: undersized tanks (2–3 m³) miss 40% potential in wet seasons; oversized tanks (10+ m³) cost EUR 3000–5000 with minimal gain beyond 60% coverage
Pitched roofs yield 15–25% more harvest than flat roofs; metal or tile surfaces outperform bitumen (0.70 vs 0.55 coefficient)
Garden irrigation demand varies 300% seasonally; simulate monthly profiles rather than annualized averages for accurate tank sizing
First-flush diverters (50–100 L capacity) essential in polluted urban areas; rural sites can reduce filter grade from 98% to 85% and save EUR 400