Water Tank Calculator
Tank capacity in litres and gallons from dimensions, with a recommended size by household
Frequently Asked Questions
Water tank sizing combines two separate calculations: the tank's physical storage capacity from its dimensions, and a recommended capacity based on household size and daily usage — and the two numbers don't always agree, since an existing or planned tank's physical size is fixed by the space available, while household need scales with the number of people actually using it.
Why the per-person guideline is a rule of thumb, not a code requirement
A commonly used household water planning guideline is roughly a fixed number of imperial gallons per person per day of storage — enough to cover normal daily use plus a buffer for supply interruptions. This is explicitly a rule of thumb, not a plumbing-code mandate, and actual need varies with household usage habits (garden watering, car washing, number of bathrooms in simultaneous use) and with how frequent and how long water supply interruptions or loadshedding-driven pump outages actually are in a specific area — a household with reliable, near-continuous supply needs less buffer storage than one dealing with regular multi-hour outages.
A worked example
A tank sized purely to physical dimensions available (say, a space-constrained rooftop location) can come out smaller than the recommended capacity for the actual household size using it — in that situation, the physical constraint and the usage recommendation are in tension, and the practical fix is usually either a taller/deeper tank within the same footprint, or an additional second tank, rather than assuming the space-constrained size is automatically adequate.
How this connects to your other property planning
Water tank capacity is one of several utility-sizing decisions that scale with household size — for the electricity side of similar planning, the Solar System Size Calculator and Electricity Bill Calculator apply the same "size to actual household usage" logic to power rather than water.
Common mistakes
Sizing a tank purely to whatever physical space is available, without checking whether that capacity actually meets the household's realistic daily need, is the most common mistake — especially in areas with frequent supply interruptions, where a marginally-sized tank leaves very little buffer before a real shortage during an extended outage.