Australia is a vast country with varied climate regions. This means that rainfall patterns can differ greatly from region to region. If you're looking to install a water tank for rainwater harvesting purposes, you need to determine what size water tank you need for your home. 

While you can always play it safe by buying the biggest and most practical tank for the available space, a more accurate way to determine the right size tank for your home is to figure out how much rain can be collected from your roof's surface beforehand. The resulting number represents the home's rainwater harvesting capacity or potential.

Several factors come into play when working out how much water can be collected from a residential roof. Continue reading to get yourself acquainted with these factors.

The average amount of rainfall you receive in your area every year

While the amount of rainfall that you receive in your specific area varies depending on the time of year, it's possible to know the average amount of rain that falls annually. The more rain falling in your area every year, the higher your property's rainwater capturing potential will be. 

If you'd like to know the exact amount of rain that falls in your area of interest, check out this useful online resource.

The size of your roof

Of course, bigger houses have more rainwater harvesting potential than smaller ones because of their larger roofs. The bigger the square footage of your roof, the more rain will fall on it and the more water it will capture. 

To determine the square footage of your roof, calculate its footprint. For a regular square or rectangular house, you can calculate the roof's footprint by simply multiplying the length times the width. The slope of the roof doesn't affect the amount of rainwater that can be collected off the roof — it only affects how fast water will run off when it rains. 

Steep-sloped roofs allow water to run off faster than less-steep, low-sloped versions. Since they cause the water to drain more slowly, low-sloped roofs increase the potential for contamination to remain on the roof's surface.

To determine the right size water tank for your home, you'll need to calculate the home's rainwater catchment capacity first — that is, how much rain can be harvested. Do this by multiplying your expected annual rainfall amount by your roof's surface area. The resulting number indicates how much water you can capture per year. Next, choose your rainwater tank size based on how much water you'll need in your home. For more information on rainwater tank sizing, feel free to consult a local expert.