How long will your tank water last?
A tank level becomes more useful when you connect it to how much water you use each day. Tankwise separates two dates: when you reach your chosen reserve, and when your estimated supply runs out.
Start with three numbers
- Your tank’s capacity in litres.
- Your current level as a percentage of that capacity.
- Your estimated household water use in litres per day.
For connected tanks, only combine their capacities if that water is actually available to the same supply. A height reading is not always the same as a volume percentage: use your tank’s volume chart for irregular shapes or horizontal cylinders.
Calculate your water remaining
Water remaining = capacity × level percentage ÷ 100
A 22,500-litre tank at 64% holds about 14,400 litres. At an assumed use of 430 litres per day, that is about 33 full days of water, with no more rain, deliveries or other losses.
Protect a planning reserve
Days to reserve = (water remaining − reserve litres) ÷ daily use
In the same example, a chosen 15% reserve is 3,375 litres. That leaves 11,025 litres above reserve, or about 25 full days at 430 litres a day. These figures illustrate the calculation; they are not recommended usage or reserve levels.
Refresh the estimate regularly
Log a new reading when you check the tank, especially after significant rain, a delivery or a change in household use. Between readings, Tankwise subtracts your assumed daily use for each elapsed calendar day. This is an estimate, not a sensor measurement.
If you estimate use from the change between two readings, choose a period without rainfall, deliveries, transfers or overflow. Otherwise the level change alone cannot tell you how much water you used. Allow for unusable water below an outlet in your planning buffer.
Try the rainwater planner →