Rainwater harvesting calculator
Each 1 mm of rain on 1 m² equals 1 liter; the tool applies a runoff coefficient for your roof surface.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter roof plan area, rainfall and runoff coefficient.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Water collected (L) = catchment area (m²) × rainfall (mm) × runoff coefficient. Cubic meters = liters ÷ 1000.
Example
Inputs: Roof plan area = 100 m², Rainfall = 100 mm, Runoff coefficient = 0.8. Result: Water collected: 8,000 L; In cubic meters: 8 m³; In US gallons: 2,113 US gal.
Limits and caveats
- The runoff coefficient is an estimate: about 0.8 to 0.9 for metal and tiled roofs, lower for rough concrete or gravel roofs.
- Light drizzle may never reach the tank, and a first-flush diverter takes a little off the total.
Questions about this tool
Why is 1 mm on 1 m² one liter?
Because 1 m² × 0.001 m = 0.001 m³, which is exactly 1 liter.
Should I use the sloped roof area?
No. Rain falls vertically, so use the plan area seen from above, building length × width.
Where do I find rainfall for my city?
From your national weather service; use the monthly or yearly average depending on what you want to estimate.