The capacity printed on a tank is not necessarily the volume you can spend on the garden. Some water may be deliberately reserved or unavailable, and the same tank may supply uses beyond irrigation. Both reduce the useful refill interval.
The Garden Water Tank Calculator calculates usable storage after an entered reserve percentage, then divides it by the combined daily draw. It assumes a full tank at the starting point and no incoming water during the calculation.
A thousand litre tank with shared use
Enter 1,000 litres of capacity and a 10% unavailable reserve. Usable water is 900 litres. With 120 litres per day for the garden and 30 litres per day for other use, total daily draw is 150 litres.
That storage lasts six days at the entered constant draw. Dividing by garden use alone would suggest 7.5 days, but that would ignore the additional 30 litres leaving the same tank each day. The combined draw is therefore worth checking before copying the result into a refill plan.
Capacity is not a current level reading
This tool does not ask how much water is presently in the tank. If a nominal 1,000 litre tank is only half full, the six day example does not describe its remaining water. Work from the storage volume actually available to your planning scenario rather than treating the tank label as a gauge.
The reserve describes the share you do not intend or cannot use. It is not an automatic estimate of outlet height or sediment. Those details need to be measured or decided for the actual tank.
Daily garden use should include the total being drawn from storage, not just a net crop requirement before irrigation losses. Water that leaves the tank but does not reach the intended root zone still reduces the stored volume.
A refill interval can change during the week
Rain, supply interruptions, different watering sessions and changes in other demand can alter the real interval. The calculator does not add incoming water or simulate varying daily withdrawals. Its days result is a constant draw comparison rather than a live tank monitor.
Use Drip Irrigation Runtime Calculator or Crop Water Budget Calculator to build a garden volume estimate, while keeping net and gross water distinct. If you collect rainfall, Rainwater Harvest Calculator estimates how much of one event can fit into the remaining tank space.
Save the storage assumption with the daily draw. Reviewing both makes a later refill estimate more useful than reusing six days after the garden or household demand has changed. Water suitability and the condition of the storage system remain separate from this quantity calculation.