Roof Drainage Flow Estimator
Rainfall depth over an area can be converted into a water volume rate. One millimetre of rain over one square metre is one litre, before the runoff factor is applied.
How to use this calculator
- Enter horizontal projected catchment area and a design rainfall intensity from a suitable local source.
- Check that units, denominator definitions and time periods are consistent before comparing scenarios.
- Run a second scenario when an assumption is uncertain.
Formula used by roof drainage capacity estimator calculator
Runoff (L/s) = catchment area (m²) × rainfall intensity (mm/hour) × runoff factor / 100 / 3,600.
- Runoff (L/s) = catchment area (m²) × rainfall intensity (mm/hour) × runoff factor / 100 / 3,600.
- Do not use this estimate alone to size life-safety drainage or prevent flooding. Use local code methods and a qualified professional when required.
- The tool performs arithmetic on the values entered and does not validate assumptions against outside records.
Worked examples
Worked example
120 m² catchment, 75 mm/hour rainfall and 85% runoff factor.
120 × 75 × 0.85 = 7,650 L/hour, or 2.125 L/second.
Estimated runoff is shown in litres per second and litres per hour.
Understanding the result
Rainfall depth over an area can be converted into a water volume rate. One millimetre of rain over one square metre is one litre, before the runoff factor is applied.
Do not use this estimate alone to size life-safety drainage or prevent flooding. Use local code methods and a qualified professional when required.
Important notes
Do not use this estimate alone to size life-safety drainage or prevent flooding. Use local code methods and a qualified professional when required.
Treat this output as an estimate tied to the stated inputs. For regulated, safety-critical, tax or professional decisions, verify applicable official guidance and specifications.
Frequently asked questions
Is roof slope included?
The entered area should represent the chosen effective catchment convention; this tool does not derive roof geometry.
Does this size gutters or downpipes?
No. It estimates runoff flow only.
How should I interpret the result?
Do not use this estimate alone to size life-safety drainage or prevent flooding. Use local code methods and a qualified professional when required.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.