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Rainwater Harvesting Calculator
Works out how much rainwater a roof can collect a year from its area and rainfall, and what size tank rides out your dry spell.
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The footprint seen from above, not the sloped surface
Enter your figures and press Calculate.
In short
Rainwater Harvesting Calculator is a free online tool on inputtesthub.com that works out how much rainwater a roof can collect a year from its area and rainfall, and what size tank rides out your dry spell. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the Rainwater Harvesting Calculator
- Enter your figures in the input fields.
- The result updates as you type, together with the full working.
- Check the breakdown table to see exactly how the number was reached.
About the Rainwater Harvesting Calculator
The Rainwater Harvesting Calculator works out how much rainwater a roof can collect a year from its area and rainfall, and what size tank rides out your dry spell. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Sizing a tank before buying one.
- Checking whether a roof can cover garden watering.
- Costing a harvesting system against the mains water it replaces.
Details
| 1 mm on 1 m² | Exactly 1 litre |
|---|---|
| Metal roof | Runoff coefficient 0.90 |
| Green roof | Only 0.35 |
| Use plan area | A pitched roof catches its own shadow |
| First flush | Discard the dirtiest early runoff |
The maths behind it
This tool uses yield = plan area in m² × rainfall in mm × runoff coefficient × (1 − losses), because 1 mm of rain on 1 m² is exactly 1 litre. Every intermediate value is shown on screen, so the result can be checked by hand.
The runoff coefficient is why a roof never delivers what falls on it: some water evaporates off a hot surface, some wets the roof and stays there, and some is deliberately dumped by a first-flush diverter to keep the dirtiest early runoff out of the tank. Metal loses the least, a green roof by far the most.
Where this tool stops
Yearly totals hide the seasonality that actually governs a harvesting system. Somewhere with 800 mm spread evenly is a completely different design problem from somewhere with 800 mm in four months, and only a month-by-month model can see that. Rainwater from a roof is not drinking water without treatment.
Explained in our guides
All guidesFrequently asked questions
Do I use the sloped roof area or the flat footprint?
The footprint. A pitched roof intercepts exactly the rain that falls on its shadow, so measuring along the slope overstates the yield by the pitch factor — about 15% on a typical 30° roof. This is the single most common error in rainwater sizing.
What size tank do I need?
Usually the dry spell decides it, not the rainfall. Multiply your daily demand by the longest dry period you want to ride out; a bigger tank than that spends most of the year part empty and stops earning its cost. The wettest-month figure is a useful cross-check for the opposite failure — a tank that overflows every winter is undersized for storage but not for supply.
Is the Rainwater Harvesting Calculator free, or is there a paid tier?
Free, unlimited, and no email address required. You can use it a hundred times a day and nothing changes.
Is the Rainwater Harvesting Calculator safe to use with private information?
No. The Rainwater Harvesting Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
How is the result calculated?
It uses yield = plan area in m² × rainfall in mm × runoff coefficient × (1 − losses), because 1 mm of rain on 1 m² is exactly 1 litre. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What do I get if I try it?
Enter a 120 m² tiled roof and 800 mm of rain and the Rainwater Harvesting Calculator returns about 73,400 litres a year after a 10% loss allowance.
What is the Rainwater Harvesting Calculator used for?
The jobs it is built for: sizing a tank before buying one. Checking whether a roof can cover garden watering. Costing a harvesting system against the mains water it replaces. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Where does the Rainwater Harvesting Calculator stop being reliable?
Yearly totals hide the seasonality that actually governs a harvesting system. Somewhere with 800 mm spread evenly is a completely different design problem from somewhere with 800 mm in four months, and only a month-by-month model can see that. Rainwater from a roof is not drinking water without treatment. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What are the key numbers for the Rainwater Harvesting Calculator?
1 mm on 1 m² is Exactly 1 litre. Metal roof is Runoff coefficient 0.90. Green roof is Only 0.35. These are printed on the page itself, so you do not have to take them on trust.
Is the number rounded?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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