First flush reduces pollutant entry, but its volume should reflect roof size and required water quality.
The key principle
Do not size a tank from annual rainfall alone. The same yearly yield can perform very differently when summer is dry, demand is high and storage is limited.
Worked example
A 100 m² roof, 50 mm rainfall, 0.9 runoff and 0.9 efficiency yield about 4,050 litres in a month: 100 × 50 × 0.9 × 0.9.
What to check before a decision
- gutter levels and safe overflow
- actual horizontal roof projection
- local rainfall and dry years, not just the average
- water quality required by the intended use
- access for cleaning, filters and pump
Limitations
The calculator does not design structures, storm drainage, non-potable plumbing or infiltration. Values must be confirmed on site.