Storage capacity
135-150 litres (35-40 gallons) per person per day is a common domestic design figure covering drinking, cooking, bathing, and general household use. Storage/autonomy days is how many days of supply you want on hand — 1 day is typical where mains supply is reliable, more where supply is intermittent.
Tank depth from footprint
Given a rectangular tank's length and width, this derives the water depth needed to hold your required capacity. Add 100-150mm (4-6in) of freeboard above the calculated water depth so the tank doesn't need to be filled right to the brim, leaving room for inlet turbulence and preventing overflow.
Frequently asked questions
How much water storage does a typical household need?
Using the common 135-150 L (35-40 gal) per person per day design figure, a household of 5 with 1 day of storage needs roughly 675-750 litres — though this varies by region, lifestyle, and whether supply is reliable enough to need less buffer.
How much freeboard should I leave above the water line?
100-150mm (4-6 inches) is a standard allowance, giving room for inlet turbulence and a margin before overflow, without needing to fill the tank completely to the top.
Does tank shape affect this calculation?
This tool assumes a rectangular tank. For a cylindrical tank, you'd use the same required volume but solve for height using the circular footprint's area (π × radius²) instead of length × width.