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Calculators / Ecology / Roof rainwater collection
Roof rainwater collection calculator
Estimate collected rainfall from a roof’s horizontal catchment area and an assumed efficiency.
On this page
Estimated collection2,000L
Estimated collection in cubic metres2m³
Calculator controls
How to calculate
Estimate collected rainfall from a roof’s horizontal catchment area and an assumed efficiency.
Estimated collection =
rain*A*c/100 LEstimated collection in cubic metres =
rain*A*c/100000 m³Worked example
- Rainfall depth (mm): 25
- Horizontal catchment area (m²): 100
- Collection efficiency (%): 80
- Estimated collection: 2,000 L
- Estimated collection in cubic metres: 2 m³
Assumptions and limits
Efficiency accounts for assumed collection losses only. First-flush diversion, overflow, evaporation, rainfall timing and water quality need separate assessment.
Results are rounded for display; calculations use unrounded values. Read our calculation methodology.
Understanding the result
Estimate collected rainfall from a roof’s horizontal catchment area and an assumed efficiency. Use horizontal roof projection rather than sloped surface area. One millimetre over one square metre equals one litre.
When this tool is useful
Use horizontal roof projection rather than sloped surface area. One millimetre over one square metre equals one litre.
Understanding your inputs
| Input | Example |
|---|---|
Rainfall depth (mm) rain | 25 |
Horizontal catchment area (m²) A | 100 |
Collection efficiency (%) c | 80 |
Frequently asked questions
Which assumptions matter for this result?
Efficiency accounts for assumed collection losses only. First-flush diversion, overflow, evaporation, rainfall timing and water quality need separate assessment.
How can I check the worked example?
Use the displayed example inputs in the stated formula. Keep each quantity in its labelled unit and compare the unrounded result before applying display rounding.