Tank Days Supply Calculator
The tool works out how much water is in the tank right now (capacity × fill %), subtracts any reserve you want to keep, and divides the remaining usable litres by your daily usage rate.
Enter Values
How to use this calculator
- Enter the tank's total capacity in litres and its current fill level as a percentage (leave fill blank to assume a full tank).
- Enter your average daily usage in litres per day; optionally add a reserve volume you want to keep in the tank.
- Read the days of supply — how long the usable water lasts at that steady rate with no top-up.
How it works
The tool works out how much water is in the tank right now (capacity × fill %), subtracts any reserve you want to keep, and divides the remaining usable litres by your daily usage rate. The core formula is: days = (capacity × fill% − reserve) ÷ daily usage. A full 5,000 L tank used at 300 L/day therefore lasts 5,000 ÷ 300 ≈ 16.7 days.
The result assumes a constant daily draw and no inflow, so it is a planning estimate rather than a guarantee. If the reserve equals or exceeds the water on hand there is no usable water, so the supply is reported as zero days. To model refills, run the calculator again with the topped-up volume, or reduce the daily usage figure to reflect water-saving measures.
Worked example
A 5,000 L tank at 100% full, using 300 L/day. Enter a tank capacity of 5,000 L, a current fill level of 100%, a daily usage of 300 L/day and no reserve. The tank holds 5,000 L of usable water, so it lasts 5,000 ÷ 300 = 16.7 days. Keep a 500 L reserve and the usable water drops to 4,500 L, giving 15 days of supply.
Common mistakes
- Mixing up capacity and current volume — capacity is the tank's full size; use the fill level field to say how full it is right now.
- Entering usage per person instead of total household or site usage — sum every user and appliance to get the true daily draw.
- Forgetting the reserve or pump-cutoff level — most pumps stop drawing before the tank is bone dry, so the last several hundred litres are often not usable.
Frequently asked questions
What daily usage figure should I use?
Use your real average daily draw in litres. A rough guide for household water is 150–200 L per person per day, but check your own meter or pump readings for an accurate number, and add irrigation, stock or pool top-ups where relevant.
Does this account for rain or a water delivery topping up the tank?
No. The estimate assumes no inflow — a steady draw-down from the current level. If you expect a refill, re-run the calculator with the new (higher) volume, or lower the daily usage to reflect the extra supply.
Why does the reserve field matter?
Many pumps have a low-level cut-off, and rural properties often keep a firefighting reserve. Water below that level is not usable for everyday supply, so subtracting the reserve gives a more realistic days-of-supply figure.
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Cite this tool
Zerdly. (2026). Tank Days Supply Calculator. Retrieved from https://www.zerdly.com/tools/tank-days-supply-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.