When to use this calculator
Estimate collected water from rainfall, roof area and a runoff factor. Does not cap output at tank capacity or model storage, first flush or overflow.
Explore the water use & conservation calculators for other questions in this topic. For a related calculation, use the Shower Water Saving Calculator: Compare water use before and after reducing flow rate for repeated showers.
PUTTING IT INTO PRACTICE
Rainwater Collection: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Rainfall (mm) | 50 |
| Catchment area (m²) | 100 |
| Collection factor (%) | 80 |
| Result | 4,000 litres |
- Check the input labels and enter the example values above.
- Apply the relationship: Litres = rainfall(mm) × area(m²) × collection factor/100.
- The result is 4,000 litres, rounded for display.
How to interpret your result
Does not cap output at tank capacity or model storage, first flush or overflow.
Use measured flow rates when possible. A rainfall volume estimate is not a storage simulation and can exceed tank capacity. Fixed water charges and sewerage charges may follow separate tariff rules.
Frequently asked questions
Can rain collected exceed tank capacity?
The geometric collection estimate can. Storage, overflow, first-flush diversion and use during rainfall need a separate tank model.
How do I use the rainwater collection calculator?
Enter rainfall (mm), catchment area (m²), collection factor (%). The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Litres = rainfall(mm) × area(m²) × collection factor/100.
What should I check before using the result?
Does not cap output at tank capacity or model storage, first flush or overflow.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
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