Wells & water/ CALCULATOR
Rainwater first-flush volume
Estimate the initial roof runoff to divert before sending rainwater toward storage.
Use the horizontal roof area that actually drains to this collection system.
Choose this from local guidance and site conditions; it is not a universal treatment value.
Count the outlets that split this connected roof area between separate diverters.
Total first-flush diversion
60.0 L
Diversion volume per outlet
30.0 L
Roof yield after diversion per additional mm
120.0 L/mm
Interpretation:Divert about 60 L before storage. If the connected outlets share the roof evenly, provide about 30 L at each diverter.
One millimetre of rain on one square metre equals one litre before collection losses. Uniform rainfall and an even split between outlets are assumed. The selected diversion depth is a planning input, not a water-safety standard. A first-flush device does not make rainwater potable; verify local rules, intended use, roof materials, treatment and maintenance.