What Size Rainwater Tank 3

OtherDIGITAL CAMERA posts explain how to calculate:


From your calculations find out the month of lowest average summer rainfall which is likely to be the peak month.

Design Case 1

If each monthly averages rainfall is above the estimated use for that month.

You will need a tank with around 10% more volume than required for  the peak month of use. This allows for evaporation, and the water that’s unavailable because its below the tank outlet.

Design Case 2

If there are  monthly averages rainfall below the estimated use for that month?

For this case you will need to add the differences in volume between the expected use and the rain for each month leading up to the peak month to the amount in Design case 1 above.

When you have the expected storage volume you can then look at storage tanks.

Not enough room…………… then you need to think about reducing your use or getting more collection area.

Bushfire Zone

If you are in an area with a bushfire risk you will need additional storage, in tanks that will be resistant to flame attack.

See this post for more information:  Bushfire Reserve


Before choosing a tank look at Round or Slimline Tanks.



  1. Austen

    Sorry, Gonna have to disagree with this 🙂

    IMHO tank size is like hard drive size, no-one ever said “My hard drive is too big, I wish I got a smaller one” !!

    We’re getting a concrete one underground, and right now we’re waiting for the engineer and builder to finish negotiations, it’ll be either 9,000l or 22,000, depending on exactly where it goes.

    With a “regular” modern suburban block, there just isn’t the room to put a decent sized one next to the house !


    1. Brian Ashworth (Post author)

      Hi Austen

      Everyone to their own opinion. 🙂

      If rainwater was my only supply on a rural block I would be looking for a bigger tank than these minimums too.

      For a suburban block I would rather reduce the water usage than spend the money getting a bigger tank. The best way of saving water is to plan a drought proof garden. For instance we didn’t water our garden from 2007-2011 even though Victoria was in drought conditions.


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    1. Brian Ashworth (Post author)

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