Start with a demand schedule
List essential and discretionary uses, average daily volume, peak rates and seasonal changes. Do not hide occupancy, irrigation or process assumptions inside a universal default.
No-refill scenario
Daily demand multiplied by the outage or autonomy period gives the basic usable volume when no source is available.
Partial-refill scenario
Subtract only refill that is credibly available during the same period. A rated source flow is not continuous refill unless operating time and reliability support it.
Reserve
Add a transparent reserve for uncertainty or operational policy. Fire storage and regulated emergency storage should remain separate unless an authority directs otherwise.
Usable versus nominal volume
Dead storage, pump-off level, sediment allowance, thermal constraints and operating setpoints can make nominal capacity larger than usable capacity.
Peak flow and delivery
A tank can hold enough daily volume but still fail to deliver the required peak flow or pressure. Check outlet, pump and pipe hydraulics separately.
Water age and quality
Oversized storage can reduce turnover. Protect inlets, vents, overflows and access; use suitable materials and maintenance, testing and treatment for the intended use.
Sensitivity cases
Compare lower refill, longer outage, higher demand and reduced usable volume. Report the assumptions with the selected capacity.
Worked example
For 2,000 L/day over 3 days, no-refill demand is 6,000 L. Reliable 5 L/min refill for 4 hours/day contributes 3,600 L; 20% reserve and 90% usable fraction lead to 3,200 L nominal storage.