Cistern size can be estimated by multiplying the depth
===Sizing Tool===
Sustainable Technologies have produced a rainwater harvesting design and costing tool specific to Ontario[http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/]. The tool is in a simple to use Excel format and is free to download.