Rainwater Days Remaining Calculator
Estimate how long stored water will last and what forecast rainfall may contribute.
What this tool calculates
It divides usable tank water by total daily household demand, then estimates possible roof capture from forecast rainfall. One millimetre of rain falling on one square metre of roof equals one litre before losses.
How to use it
Estimate the current volume from a gauge or tank dimensions, protect a sensible reserve, and enter actual household use if known. Only count roof sections connected to the tank and reduce efficiency for first-flush diversion, leaks, gutter losses and overflow.
Useful examples
- Deciding when an off-grid household should order a water delivery.
- Testing how shorter showers or fewer washing loads extend supply.
- Estimating whether forecast rain is likely to make a meaningful difference.
Important limits
Forecast rainfall may miss the property and intense rain can overflow gutters or tanks. Water quality, pump intake height and sediment can reduce usable volume. Keep an emergency margin.
Why daily demand matters
Two households with the same tank level can have very different security. Toilets, showers, washing machines, leaks, animals and garden use all affect demand. A short meter-reading trial gives a better daily estimate than a generic average.
Worked example
A tank holding 5,000 litres with a 500-litre reserve has 4,500 usable litres. Two people using 80 litres each per day have about 28.1 days remaining. Twenty millimetres across 100 square metres at 80% efficiency could add about 1,600 litres, extending the estimate by ten days if the tank has room.