Tank & Water Budget Simulator
Enter your roof, rainfall, tank and water use to see month by month when the tank fills, overflows or runs dry, how much of your use it meets, and the smallest tank that would cover 90 percent.
A rainwater tank is only as good as the months it runs dry in. The annual harvest figure from a roof is easy, but a tank that overflows in March and is empty by November is not meeting your needs. This simulator runs your tank month by month: rain in, water used out, how full it ends each month, and the first month it runs dry. Change the tank size, the roof or your habits and watch the dry months move.
Tank & Water Budget Simulator
Pick your region or enter your own rainfall and pattern. Rainfall by month uses typical patterns for each region and an average year, not a forecast, and a real tank can run dry between monthly steps in a dry spell. Treat dry months as warnings. The Rainwater Harvesting Calculator covers annual totals and tank sizing.
Water collected a year: —
Water you use a year: —
Share of your use met by the tank: —
Months the tank runs short: —
First month it runs dry: —
Overflow lost a year: —
Smallest tank to meet 90% of use: —
Full guide, more detail, and a printable version at https://hollyshomestead.com.au/tools/tank-water-budget-simulator.html
How the Simulation Works
Each month, rain in equals roof area times that month’s rainfall times your collection efficiency, because one millimetre of rain on one square metre is one litre. Water out is your indoor use, stock water and garden watering. Garden watering is highest in hot, dry months and falls away when it rains. The tank cannot hold more than its size, so extra rain is lost as overflow, and it cannot go below empty, so any shortfall is water you would have to buy or find elsewhere. The simulator runs a few years from your starting level, then reports a typical year.
Rainfall by month comes from five regional templates scaled to your annual rainfall: even, winter-dominant, summer-dominant, a summer-to-autumn peak, and monsoon. Real years vary a lot. A drought year can deliver half the average, so a tank that just works in an average year is tight in a dry one.
Making the Tank Last
Three levers move the result. A bigger roof or an extra tank catches more of the wet months, mulching and drip irrigation (see drip irrigation and mulching) cut the garden’s share, and a bigger tank only helps if there is a roof large enough to fill it. Check the tank sizing guide for placement, first flush devices and overflow management.