Roof rainwater calculator
See how many liters of rainwater you can collect from your roof in a year.
Your result
Water a year—
Average a month—
First flush per shower—
These are planning figures. Results depend on your climate, soil and experience.
How it is calculated
- 1 mm of rain on 1 m² of roof gives 1 liter.
- In practice you collect 60–90 %, depending on the roof.
- Measure the roof area as seen from above, not along the slope.
- Let the first 0.5–1 liter per m² of roof run off in every shower.
- Rainwater is not drinkable without filtering, disinfection and a lab test.
In the guide: Rainwater harvesting: gutters, cisterns and tanks
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