Estimate how many days your water will last
Enter your total tank capacity, current volume percentage and estimated daily use. The calculator shows two different planning windows: time until your chosen reserve and time until the estimated tank volume reaches zero.
Water remaining = tank capacity × level percentage ÷ 100
Days until empty = water remaining ÷ daily use
Days to reserve = max(0, water remaining − reserve litres) ÷ daily use
A worked example
A 22,500-litre tank at 64% contains 14,400 litres. If daily use is 430 litres, that gives approximately 33.5 days until empty with no more inflow. A chosen 15% reserve protects 3,375 litres, leaving 11,025 litres above reserve: approximately 25.6 days.
For practical planning, round the days down and allow an additional buffer. These example figures explain the maths; they are not a recommendation for your household’s usage or reserve.
Improve your estimate
- Use a recent level reading. A reading taken before heavy use or a delivery can be misleading.
- Check whether a depth percentage accurately represents volume. It does for a uniform vertical tank; it does not generally do so for a horizontal cylinder.
- Use a realistic daily consumption estimate. To infer use from falling levels, choose a period without rainfall, deliveries, transfers or overflow.
- Include water below an outlet and any water you need to protect in your planning allowance.
Common questions
Does this include future rain?
No. This is a no-rain baseline so you can see the dry-weather planning window. Use the Tankwise planner to compare rainfall and delivery scenarios across 30 days.
What if the current level is already below reserve?
Days to reserve is zero. The tool still shows the estimated time to empty, but that does not mean your protected buffer remains available. Check your level and refill options.
Can I combine two tanks?
Only combine capacities and stored water if both tanks supply the same usable system. A disconnected tank or one with a different outlet arrangement may not contribute to the supply you are planning.