Rainwater Harvesting Calculator (Roof Collection)
Enter roof area and rainfall to see how much water you can harvest.
Your numbers
Yearly or for one storm
%
Your results
Water collected (litres)
Water collected (litres)
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Cubic metres
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US gallons
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Tank size for the dry spell (L)
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Days of use from this rainfall
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Rainwater Harvesting Calculator (Roof Collection) by TheCalcTools
How the Rainwater Harvesting Calculator (Roof Collection) works
One millimetre of rain on one square metre is one litre of water. Multiply roof area by rainfall, then by a runoff coefficient for the roof material, and subtract losses from the first-flush diverter and filters.
Use the roof’s footprint (the area seen from above), not the sloped surface area.
The formula
Litres = area (m²) × rainfall (mm) × runoff coefficient × (1 − losses)
Worked example
Using the default values shown in the calculator:
| Roof footprint area | 100 |
|---|---|
| Rainfall (mm) | 800 |
| Roof type (runoff coefficient) | Tiles or concrete (0.85) |
| First-flush and filter losses | 10 % |
| Water use from the tank per day (L) | 150 |
| Dry days the tank should cover | 30 |
- 100 m² × 800 mm × 0.85 × 0.9 = 61,200 litres a year.
- A 30-day dry spell at 150 L a day needs a 4,500 L tank.
Result:
| Water collected (litres) | 61,200 |
|---|---|
| Cubic metres | 61.2 |
| US gallons | 16,167 |
| Tank size for the dry spell (L) | 4,500 |
| Days of use from this rainfall | 408 |
Frequently asked questions
How much water does an inch of rain give from 1,000 sq ft?
About 623 US gallons before losses, the common rule of thumb.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.