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Choosing the size of your tank
 
Factors to consider
Things you need to consider when determining the size of your tank or storage vessel include:
1. Rainfall - how much rainfall you get in your area,
2. Roof area - how large the catchment surface is,
3. Water usage - what you intend to use rainwater for, and how much you expect to use,
4. Site characteristics - how much space you have, and the location you intend to install the tank or storage vessel.
   
 
Calculating the appropriate size

Use the following formula:
Roof Area  X  Annual Rainfall  =  Maximum Available for Capture

Then determine:
Most Days without Rain  X  Average Daily Water Use  = Storage Volume Required on Day One of a Dry Spell

If for example, the most days without rain was 40 days, and the average daily usage was 400L, the volume on hand on Day One of 40 days would ahve to be 16,000L.

It is very difficult to predict the required volume, because often a dry period will start when the tank is not full. However, this method provides a guideline, and you can add a safety factor on top of this requriement.

When calculating how big the storage capacity should be, plan the tank area so that extra storage can be added later if required.
Location for the tank
It is important to consider the space you have available, whether the vessel is to be installed above or below ground, below the roof-line or above it (eg. on a sloping site), whether you will have one or more tanks - connected or installed as separate systems, and what ground surface preparation is required.
 
Safer than mains water?
Rainwater Research
Ideal uses for rainwater
Reasons for using rainwater
How to Create a Complete System
Water storage and tank selection
Types of water storage vessels
Choosing the size of your tank
Average rainfall
Roof surface & area
Water usage
'Wet' and 'Dry' systems
Pump selection
Recommended systems
Regulation and Rebates
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