Free tool
Rainwater tank calculator
Roof area and rainfall in, litres and a tank size out.
One millimetre of rain falling on one square metre of roof is one litre of water. That single fact is the whole calculation, and it is the one most people have never been told.
The sum
Roof area in square metres times annual rainfall in millimetres gives you litres. A 200 square metre roof under 600mm of rain a year sees 120,000 litres land on it.
You will not collect all of it. First flush diverters throw the first few minutes away, gutters overflow in heavy falls, some evaporates off a hot roof in a light shower, and some blows off entirely. Fifteen per cent is a reasonable working loss, which leaves about 102,000 litres.
Roof area means the footprint
Use the plan area, the outline of the roof looking down at it, not the sloped surface area. Rain falls vertically, so a steep roof and a flat roof with the same footprint catch the same water. Only count the parts that actually drain to the tank. A shed roof plumbed in is real catchment, a verandah that drains to the garden is not.
Size for the dry spell, not the year
This is where people get it wrong. The annual total tells you whether the roof can keep up in an average year. It tells you nothing about whether you run out in February.
Across inland New South Wales and Victoria, a run of two or three months with almost nothing is ordinary. So the tank has to hold enough to bridge the gap, not enough to hold the year's rain. A household using 240 litres a day needs about 21,600 litres to cover ninety days, which is why 22,500 litre tanks are so common on rural blocks.
Rainfall varies more than the average suggests
The annual average for a place like Wagga or Wodonga hides a lot. Some years come in well under, and the dry years are exactly the years the tank matters. If the number above is close, size up, because the cost of a bigger tank is small next to the cost of carting water.
Your local council or the Bureau of Meteorology has the long term figures for your postcode, and the median is a more useful number than the mean for this purpose.
What this does not cover
Drinking water quality, first flush and filtration, pump sizing, and whether a tank is required or subsidised where you are. Requirements differ between New South Wales and Victoria and between councils, and new builds are often covered by separate rules again.
Common questions
How much rainwater can I collect from my roof?
One millimetre of rain on one square metre of roof is one litre. Multiply your roof footprint in square metres by the annual rainfall in millimetres, then take off about fifteen per cent for first flush, overflow and evaporation. A 200 square metre roof under 600mm collects roughly 102,000 litres a year.
Do I use the roof area or the roof footprint?
The footprint, which is the outline of the roof seen from above. Rain falls vertically, so a steep roof catches no more water than a flat one covering the same ground. Only count the sections that actually drain to the tank.
What size rainwater tank do I need?
Size it to bridge the dry spell rather than to hold the year's rainfall. Inland New South Wales and Victoria regularly go two or three months with almost nothing, so a household using 240 litres a day needs somewhere near 21,600 litres to cover ninety days. That is why 22,500 litre tanks are so common on rural blocks.
Is the annual average rainfall a safe number to plan on?
Not on its own. The average hides considerable variation, and the dry years are precisely the years the tank matters. If the calculation comes out close to your usage, size up. The Bureau of Meteorology holds long term figures by location, and the median is more useful than the mean here.