Rainwater Harvesting & Tank Sizing Calculator
Rainwater harvesting calculator — enter your roof area and city to see how many litres you could collect a year, a month-by-month harvest chart, a tank fill simulation showing how much demand it meets and how much overflows, and the water-bill savings. ASEAN rainfall built in. Runs in your browser.
Rainwater Harvesting & Tank Sizing Calculator
How to Use the Rainwater Harvesting Calculator
Enter your roof and city
Set the roof catchment area and pick your city for built-in rainfall, or choose Custom and type your annual rainfall.
Choose roof type
Select the roof material so the runoff coefficient reflects how much rain actually reaches the tank.
Set tank and demand
Enter a tank size and your daily water demand. The simulation shows how much demand is met and how much overflows month by month.
Read savings
Enter your water price to see the yearly bill saving, and compare tank sizes to find the sweet spot.
Harvesting Rain from Your Roof
Rainwater harvesting is one of the oldest and simplest pieces of green infrastructure, and in a high-rainfall region it can supply a surprising share of a household’s non-drinking water. The arithmetic at its heart is elegant: because one millimetre of rain falling on one square metre of surface delivers exactly one litre, the water your roof can collect is just its catchment area multiplied by the rainfall, scaled down by a runoff coefficient that accounts for the rain that never makes it to the tank — splashed off, evaporated, soaked into the surface, or sent to waste by the first-flush diverter that discards the dirty initial runoff. A smooth metal roof passes on around ninety per cent of what lands on it; tiles and rougher surfaces a little less. Multiply it out and a typical suburban roof in a monsoon climate can yield well over a hundred thousand litres a year.
But an annual total hides the real design challenge, which is timing. Rain does not arrive evenly: monsoon-driven climates pour most of their water into a few months and leave others dry. A tank is the buffer that bridges that gap, storing the wet-season surplus for the dry spells. Size it too small and it brims over and spills during downpours, wasting the very abundance you are trying to capture; size it too large and you have paid for capacity that sits empty most of the year. This is why the tool does not stop at an annual figure. It runs a month-by-month simulation, filling the tank with each month’s harvest, drawing down your daily demand, and spilling any excess once the tank is full, so you can see exactly how much of your demand the system actually meets and how much overflows. That “demand met” percentage — not the headline litres — is the number that tells you whether a given tank is worth it.
What you do with the water shapes the whole design. Harvested rain is naturally soft and well suited to irrigation, flushing toilets, washing floors and vehicles, and laundry — the bulk of household water use that does not need to be drinking quality. Diverting those uses to rainwater cuts mains consumption and the bill that comes with it, and the tool estimates that saving from the volume actually used and the water price you enter. Using harvested rain for drinking or cooking is possible but demands proper filtration and disinfection to meet health standards, so most home systems sensibly keep it to non-potable duty. Everything here is computed in your browser from transparent factors, so you can experiment freely with roof type, tank size and demand to find a system that fits your rainfall and your needs.
The annual rainfall total tells you the prize; the month-by-month pattern tells you how big a tank you need to claim it.
10 Facts About Rainwater Harvesting
1 mm of rain on 1 m² of roof = 1 litre.
A runoff coefficient discounts splash and evaporation losses.
Metal roofs shed water better (~0.9) than tiles (~0.85).
Singapore averages over 2,300 mm of rain a year.
A tank too small overflows in the wet season.
A tank too large sits half-empty and wastes money.
Harvested rain suits flushing, washing and irrigation.
Rainwater is soft — good for plants and laundry.
Monsoon rainfall is uneven across the months.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- The harvest is simply roof catchment area multiplied by rainfall multiplied by a runoff coefficient. Because one millimetre of rain falling on one square metre yields one litre, a 100 m² roof under 2,000 mm of annual rain with an 0.85 runoff coefficient collects roughly 170,000 litres a year. Enter your roof area and city and the tool does this for every month.
- It is the fraction of rain landing on the roof that actually reaches your tank, after losses to splashing, evaporation, wetting the surface and the first-flush diverter. Smooth metal roofs are efficient at around 0.9, tiled roofs about 0.85, and flat concrete a little lower. Pick the roof type that matches yours.
- The tank has to bridge the gap between when rain falls and when you use the water. The built-in simulation runs your harvest and daily demand month by month and reports how much of your demand the tank meets and how much overflows. Try different tank sizes: a bigger tank captures more of the wet-season surplus for dry spells, up to the point where extra capacity just sits empty.
- It is the share of your water demand that the harvested rain can actually supply over the year, given your tank size. A small tank in a monsoon climate might overflow in wet months yet run dry in dry months, so even with plenty of total rainfall the demand-met figure can be well below 100%. It is the most honest measure of how useful the system is.
- Typically non-potable uses: garden and landscape irrigation, flushing toilets, washing floors and vehicles, and laundry. Rainwater is naturally soft, which plants and washing machines like. For any drinking or cooking use it must be properly filtered and disinfected to meet local health requirements.
- The tool includes monthly climatological averages for several ASEAN cities. If your location is not listed, choose “Custom” and enter your own annual rainfall, which the tool spreads evenly across the year. For the most accurate result, use long-term local rainfall figures.
- Every litre of demand met by rainwater is a litre you do not buy from the utility. The tool multiplies the usable volume in cubic metres by the water price you enter to estimate the annual saving in your currency. Enter your actual tariff for an accurate figure.
- Because rainfall is uneven, especially in monsoon climates, the month-by-month chart shows when the surplus arrives and when supply is tight. That pattern, not just the annual total, determines the right tank size and how much demand you can realistically meet.
- They are good planning estimates. Real yield depends on your actual rainfall, roof condition, gutter and filter design, first-flush losses and how steady your demand is. Use the tool to size sensibly and understand the trade-offs, then refine with local data.
- Completely free, with no account or usage limit. It runs entirely in your browser, collects no data, and works offline once the page has loaded.
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