The result is an estimate, not an engineering design
The calculator's recommendation is a starting point, not an engineering design. It is built from typical household usage figures and average monthly rainfall for your region. Rainfall varies from year to year, and the model works in whole-month steps, so a multi-week dry spell inside a month that balances on average is not captured. If you want to see how the number is worked out, and the model's limitations stated plainly, read the methodology.
This site does not sell tanks
This site does not sell tanks. It is not affiliated with, owned by, or sponsored by any tank manufacturer, and no figure on this site is sized to fit any manufacturer's catalogue.
Rules vary by council
Consent requirements, firefighting water reserve requirements, and drinking-water rules differ between councils and regions, and they change. Checking what applies to your property is your own responsibility — check with your local council before you buy or install anything. Where the guides on this site quote a regulatory figure, it is marked for verification and should be confirmed with your council before you rely on it.
Confirm before buying
Before you buy anything, get a supplier or a licensed plumber to confirm the tank size and the plumbing details for your property. A tank sized on paper against average rainfall can still be the wrong choice for your site, your soil, and your planned use. You can find verified suppliers in our supplier directory.