Rainwater Collection Calculator
Litres collected from your roof.
Enter your values
Enter the roof area in m².
Enter the rainfall in mm.
Enter the collection efficiency in %.
Enter the daily water use in L.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Water collected
- 3,060 litres
- In gallons
- 808
- Days of supply
- 51
- Annual estimate (×12)
- 36,720 litres
Quick answer
One millimetre of rain on one square metre of roof yields one litre of water.
Litres = area × rainfall × efficiency
At a glance
| What it does | Litres collected from your roof. |
|---|---|
| Category | Science |
| Inputs needed | Roof area, Rainfall, Collection efficiency, Daily water use |
| Main output | Water collected |
| Formula | Litres = area × rainfall × efficiency |
| Cost | Free — no sign-up, no download |
How to use the Rainwater Collection Calculator
- 1Enter your roof's plan area.
- 2Enter rainfall for the period.
- 3Adjust efficiency for gutter and filter losses.
Inputs explained
- Roof area(m²)
- Enter the roof area in m².
- Rainfall(mm)
- Enter the rainfall in mm.
- Collection efficiency(%)
- Enter the collection efficiency in %.
- Daily water use(L)
- Enter the daily water use in L.
Worked example
Using the values the calculator loads with:
Inputs
- Roof area80 m²
- Rainfall45 mm
- Collection efficiency85 %
- Daily water use60 L
Results
- Water collected3,060 litres
- In gallons808
- Days of supply51
- Annual estimate (×12)36,720 litres
Frequently asked questions
Is roof area the sloped area?
No — use the flat footprint area for rainfall capture.
Is rainwater drinkable?
Not without proper filtration and treatment.
What do I need to enter into the Rainwater Collection Calculator?
Just 4 values: roof area, rainfall, collection efficiency, daily water use. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Rainwater Collection Calculator work out the answer?
One millimetre of rain on one square metre of roof yields one litre of water. It applies the formula Litres = area × rainfall × efficiency and shows the working so you can check each step by hand.
Is the Rainwater Collection Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.