The average annual rainfall for Ballarat is 691 ml.  To find out the average annual rainfall for your town in Australia visit the Climate Statistics for Australian Sites webpage on the Australian Bureau of Meteorology's website.  People living outside of Australia will probably be able to find their town's annual average rainfall by visiting their own country's weather bureau website or by contacting their local agricultural department.

You need to calculate your roof area.  The formula for this is :-
 
Using my garage's dimensions as a sample the calculation is :-

Answer : 8.8 square metres.


Not every drop of water landing on your roof will runoff down the guttering drainpipes.  A thin residue will be left on the roof after rain and will eventually evaporate in the sun. 

For metal and tiled roofs assume a coefficient runoff percentage of 90% or .9.

First flush devices are designed to siphon of the first rainwater that falls on your roof.  They are used to reduce the amount of dirt and bird droppings that go into water tanks.  If  you have a first flush device fitted to your downpipe you will have to subtract the amount of water that is diverted every time it rains by the number of times it is likely to rain.  Different brands of first flush devices siphon off different amounts of water. 

Assuming an average of 48 rainfall events each year and your device diverts 20 litres of water each time it rains the calculation for lost water from a first flush device would be :-

So if you have a first flush device fitted then you would have to subtract 960 litres from your rainwater calculation.



The rainwater calculation for roof runoff without a first flush water device is :-
Using my garage roof dimensions and Ballarat's annual average rainfall as a sample the calculation is :-
So I should expect the annual discharge to my garage tank to be 5,472 Litres.

If you have a first flush water device fitted than you have to subtract the amount of diverted water.  So the sample calculation is :-