Rainwater Harvesting Calculator
We have created this calculator below to help you to determine how much water you can catch each year into your rainwater tank.
Add in your Roof Size & Annual Rainfall
Find the average annual rainfall for your area: Weather Zone
How Much Water Can You Harvest from Your Roof?
How much rainwater you can actually harvest from your roof. Fortunately, there’s a rule of thumb for that too. Each square metre of roof space collects around 1 litre of water for everyone millimetre of rainfall.
So, if hypothetically you have 10 millimetres of rain in a storm, and 150 square metres of roof space actually available to collect rainwater, you can then collect 1500 litres of rainwater from that storm (150sq metres x10 millimetres x 1 litre).
To average things out over a longer period of time, you can actually look up the monthly rainfall pattern for your geographical area – for example, let’s say 100mm per month on average over a 12-month period.
The average area for rainwater catchment for Australian households:
- 100-150 ㎡ – Small House
- 150-200 ㎡ – Medium House
- 200 ㎡ + Large House
Don’t want to get out a pen and paper, or open up Calc? Can’t blame you. Fortunately, we’ve done the hard yards and automated that process with our handy rainwater harvesting calculator.