What this tool does
Convert the project’s local first-flush criterion into an installed diversion volume without implying that one rule is universal.
When to use it
Use after identifying the roof area that actually drains to the tank and finding the applicable guidance for location, roof condition and end use.
Inputs and supported units
Each labelled field explains the quantity it expects. Unit-system changes convert existing numeric entries once and keep calculations on a single SI internal basis. Supported shared units include L/min and GPM, metres and feet, kPa and psi, millimetres and inches, kW and hp, and °C and °F as relevant.
Calculation or decision method
How to interpret the result
Read the primary result with the supporting breakdown and warning. A plausible number is not evidence that every input, operating case or design constraint has been included.
Worked example
A 150 m² roof at 0.2 mm diversion depth requires 30 L. The same result is expressed as 0.2 L/m².
Assumptions
Catchment area is horizontal plan area and one consistent local rule has been chosen.
Limitations
The result does not predict contaminant removal, storm reset/drain behavior, maintenance or harvest loss across a rainfall record.