How Long Will It Take To Fill Or Empty My Tank?

How Long Will It Take Before My Tank Fills Up?

Two of the main factors that influence how quickly your tank will fill are the size of your roof and the amount of rain that falls in your area. As a general guide, every 1mm of water that falls on 1m2 of catchment space will yield 1 litre of water.

The Australian Bureau of Meteorology reports average monthly and annual rainfall data across all areas of Australia for you to establish the average rainfall in your area. It’s also important to remember that rainfall in Australia doesn’t fall at a consistent amount each day (thankfully!) and that the varied rate of rainfall means that the tank may fill and overflow in one day and then there will be no more rain for an extended period of time. For most populated areas in Australia, the average annual rainfall is above 600mm, with the majority of coastal areas having a minimum of 1000mm per annum. This would be indicative of rainfall of between 50mm – 100mm per month.

Your catchment space is the area of your roof and/or other surfaces that will feed water into your tank in square metres.

To calculate how quickly your tank will fill, simply multiply the area of the catchment space by the average monthly rainfall. For example, for a catchment area of 200m2 and an average monthly rainfall of 75mm, this would result in 15,000 litres in one month.

How Much Water Will I Use From My Tank?

An important part of calculating how fast your tank will empty is the number of uses for your stored rainwater and how often or extensively they are utilised. For example, a rainwater tank used solely for flushing toilets in a two-toilet house using 20 litres per flush, will use around 150 litres per day. A household that uses stored rainwater solely for watering a large garden will have increased usage in the summer months (due to a combination of higher evaporation and lower rainfall) but will require approximately 10 litres per square metre of garden per week.

Tank Filling and Emptying