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Water Calculator

Water Calculator

As a rule of thumb you need to have enough water storage to hold 250mm/10” of rain on your roof area. The guide below will capture this amount using the most cost effective tank configurations. This is a guide only.

Roof Area (m2)

Average Annual Rainfall (mm)

Found out your local average annual rainfall HERE

Amount of Rain Harvested (litres)
For large capacity requirements - use multiple connected tanks. Assuming the tank can fill 3 times per year, your minimum capacity requirements are:
View Tanks
Here's how to find average local rainfall:
  1. Visit the BoM website: Use this link
    Access the rainfall section of the Bureau of Meteorology (BoM) website.
  2. Choose your area:
    Select the state or region for which you want to find the average rainfall.
  3. Select your period:
    Choose the time period for which you want to see the average rainfall, such as monthly, seasonal, or annual data.
  4. View the data:
    The BoM will provide a map or table showing the average rainfall in millimeters over a 30-year period.
  5. Interpret the data:
    Use the map or table to determine the average rainfall for your specific location or area of interest. 

FAQs

Find quick answers to common questions. For further assistance, please contact our customer service team.

How do I calculate how much water I can collect from my roof?

You can use the formula:

Roof area (m²) × Annual rainfall (mm) × 0.9 = litres collected per year.

The 0.9 accounts for runoff losses. Our calculator does this for you — just enter your roof size and postcode.

What size water tank do I need for my property?

That depends on how you plan to use the water — for garden use, household supply, stock water or all of the above. Read our blog - How much rainwater storage do I really need?

Can I connect multiple tanks together if I need more capacity later?

Yes — RapidPlas Tanks are come with 2 BSP outlets as standard so adding additional tanks is easy. However, the primary tank will need to be empty to connect to the new tank. We can help you plan for expansion with linking kits and accessories.

Can I drink rainwater from my tank?

Yes — with the right setup. You’ll need a first-flush diverter and a suitable filter system, we carry a range of filters depending on your needs. Check out our range of filters here. Read about the health benefits of using filtered rainwater

Do I need a pump with my tank?

If your tank is supplying water to your house or a pressurised garden system, then yes, a pump is typically needed. We offer packages that include a compatible pump and accessories. See our pump and tank packages here. Read about Choosing the right pump for your rainwater tank.

What maintenance does a rainwater tank need?

Rainwater tanks are low-maintenance, but we recommend:

  • Checking gutters and strainers regularly
  • Cleaning the tank inlet strainer
  • Inspecting for sediment buildup every few years

Read about the importance of tank maintenance here

What happens when the tank is full?

Once your tank reaches capacity, the overflow system directs excess water safely away — usually back into stormwater drains or onto a garden area.

Read bout plumbing the overflow away from the tank here

Do you deliver tanks to my area?

We deliver across much of NSW and SE QLD — check your postcode on our enquiry form or give us a call to confirm. Delivery is free in our core zones for large tanks 9,000L and above.

Do you install the tank too?

We don’t install tanks, we recommend you contact a local installer or plumber, or talk to your closest RapidPlas dealer for a referral. Most installations are straightforward and can be done by a local tradesperson.

What kind of base do I need for my water tank?

A level, compacted base is essential — sand, crusher dust or a concrete slab are common options. We’ve prepared a base preparation guide with your order to help you get it right. You can find it here

How long will delivery take once I order?

That depends on your location and the tank type — typically 2 to 4 weeks. We'll confirm your delivery window once you've filled out your delivery check list after the tank has been ordered.

What do I need to know about the delivery of my rainwater tank?

We have information on the website and a video outlining what to expect with your tank delivery. Watch here