Plan when to order and how much to buy
Work backwards from your reserve date using your supplier’s lead time. Then estimate how much water will remain at arrival and how much space will be available for the delivery.
Days until order point = max(0, days to reserve − delivery lead time)
Water at arrival = max(0, current water − daily use × lead time)
Delivery needed = max(0, target litres − water at arrival)
The volume calculation assumes you order now and the delivery arrives after the entered lead time. It is separate from the latest suggested order point. If you wait before ordering, update the current level and recalculate the delivery volume.
A worked example
Start with 14,400 litres in a 22,500-litre tank, using 430 litres per day. A seven-day lead time uses an estimated 3,010 litres, leaving 11,390 litres at arrival. A 90% target is 20,250 litres, so the estimated refill needed is 8,860 litres.
There would be 11,110 litres of available space. A 10,000-litre delivery fits under these assumptions and leaves 21,390 litres, around 95.1% full. Recheck the actual level before confirming the load.
Do not ignore a supply gap
If the tank would empty before the delivery arrives, the calculator reports unmet demand. It does not subtract water below zero or imply your household can keep drawing from an empty tank. Contact a supplier about an earlier delivery or arrange another source.
Before booking
- Confirm the lead time, minimum load and available delivery dates.
- Check the actual level and spare tank capacity, including any rainfall since your last reading.
- Tell the supplier about access, gates, driveway restrictions and hose distance.
- Confirm that the delivered water suits your intended use.
Common questions
Does this reserve percentage guarantee a safe buffer?
No. You choose the reserve. Actual demand, unusable water and delays can vary. The calculator does not certify firefighting capacity or another required reserve.
What if my delivery amount is larger than the available space?
The excess is shown as potential overflow. Ask about a smaller delivery or another appropriate storage arrangement. Do not assume the whole load can be retained.
Does this book a water delivery?
No. It is a planning calculation. Use your own supplier’s availability and confirm the final amount with them.