Estimate one rainfall event and the amount a tank can receive.
Horizontal projected area that drains to the tank.
Choose a rainfall event, not an annual average.
Explicit assumption for losses before reaching storage.
Capacity excluding unusable dead storage.
Must not exceed usable capacity.
Potential collection
1,600L
Added to storage
1,000L
Surplus beyond free space
600L
Single event, uniform collection factor and no withdrawals during rain. Does not size gutters or drainage, predict dry-period reliability, or establish water quality.