Rain on a roof and new water retained in a tank are different volumes. Collection losses, a first flush diversion and the water already in storage can each reduce what is available after a rain event.
The Rainwater Harvest Calculator follows that sequence. It estimates collection from horizontal catchment area, rain depth and a collection share, subtracts the first flush, then fits the remaining volume into the tank's free space.
An event that fills the remaining space
Take an 80 m² horizontal roof catchment, 20 mm of rain and an 80% collection share. The estimated collection before first flush is 1,280 litres. Subtract a 40 litre first flush and 1,240 litres remain for storage.
A 1,000 litre tank already holding 300 litres has only 700 litres free. The calculator retains 700 litres from the event, reports 540 litres of overflow after first flush and finishes with 1,000 litres in the tank.
| Event stage | Volume |
|---|---|
| Collected before first flush | 1,280 L |
| Available after 40 L first flush | 1,240 L |
| New water retained | 700 L |
| Overflow after first flush | 540 L |
Use the catchment basis shown in the field
The area input is the horizontal roof catchment served by the collection arrangement. It is not automatically the sloping surface area of a pitched roof, and it should not include a section whose drainage is not connected to this tank.
The collection share is your assumption about the event. The example of 80% is not a measured efficiency for your roof. A first flush volume is then removed separately, so avoid using a collection percentage that has already deducted the same diversion unless that is your deliberate calculation basis.
If first flush exceeds the collected volume, water remaining for storage becomes zero. Existing storage larger than tank capacity is rejected because it cannot represent the stated tank state.
Storage arithmetic does not assess water quality
A retained litre figure does not establish that collected water is suitable for a particular use. Roof materials, contamination, filters and storage conditions are not evaluated here. The tool also does not size gutters or calculate peak storm flow.
For a garden supply estimate, use the newly retained or total usable storage with Garden Water Tank Calculator and the appropriate daily draw. Keep an actual level reading and any unavailable reserve separate so that the next calculation does not quietly treat all nominal tank capacity as spendable water.