The volume of water you can harvest largely depends upon your rainfall (measured in mm over time) and your roof size (measured in square metres).
National or local weather services such as the Bureau of Meteorology in Australia can provide historical rainfall data for your area, while you can calculate your roof size by looking at your building plans or using Google Maps.
Past water bills, estimates based on the factors above or data from meters and gauges can all help you more accurately forecast your water needs.
If you experience seasonal rainfall or your peak rainfall and water use times differ, you’ll want bigger tanks to capture enough water for drier times.
You will also want to ensure you have connected as much roof area as is practical to your tank.
Balancing the volume of rainwater you need with the volume you can harvest will inform your decisions regarding tank numbers and size. You should also design your Rain Harvesting system with appropriate products to minimise water loss.