Verify the inputs
Measure or document each input. If a value is only an estimate, record its source and test a conservative alternative.
Wells & water/ CALCULATOR
Rainwater first-flush volume
Estimate the initial roof runoff to divert before sending rainwater toward storage.
FROM ESTIMATE TO ACTION
Use the result, not just the number.
Keep the calculation traceable
Save the input link or copied result together with the date, units and assumptions. A number without its inputs cannot be checked later.
Confirm the decision
Compare the estimate with the related guide, current product data and requirements that apply at your location before buying or building.
Inputs to verify
- Connected projected roof area (m²)
- Use the horizontal roof area that actually drains to this collection system.
- Selected first-flush depth (mm)
- Choose this from local guidance and site conditions; it is not a universal treatment value.
- Connected diversion outlets (outlets)
- Count the outlets that split this connected roof area between separate diverters.