Plan water storage capacity
Start with daily demand, reserve days and the fraction of tank capacity you can actually draw.
Back to calculator ↑Separate uses
Enter people and litres per person per day, then add garden or cleaning use. Meter records can improve the assumption.
Enter use for the days you actually plan to reserve. It need not equal a normal day: some households can pause garden watering, while others may have more occupants. Review several meter readings and separate essential from deferrable uses so litres per person and other demand reflect the intended scenario.
Allow for unusable water
Nominal tank capacity may not be fully drawable due to outlet or pump level. Divide needed reserve by usable fraction, then round tank count upward.
Usable fraction depends on outlet height, pump minimum water level and tank shape. The example 90% merely reserves 10%; it is not a specification for any tank model. If the actual fraction is 80%, a 900 L usable reserve needs 1,125 L nominal capacity.
Worked default example
Four people × 100 L plus 50 L other use = 450 L/day. Two days require 900 L usable. At 90% usable capacity, nominal capacity is 1,000 L, or one entered 1,000 L tank.
At the default 1,000 L nominal requirement and entered 1,000 L tank, one tank is enough by volume. Changing the entered tank size to 750 L rounds up to two, providing 1,500 L nominal. Before selecting multiple tanks, check site space and the manufacturer-approved connection arrangement.
Check the real installation
Tank count estimates volume only; it does not check foundation loads, pumps, water quality, refill or building requirements. Confirm with the product and installer.
A full tank is heavy; a volume estimate cannot establish that a floor or base can carry it. Storage also involves cleanliness, covers, circulation and maintenance. For drinking water, obtain water-quality and treatment advice appropriate to the actual source and installation.
When comparing tank sizes, do not choose solely by the smallest count. One large tank and several small tanks can differ in space, maintenance and resilience if one tank is unavailable. This tool rounds only by volume; check the site layout and piping separately.
Common questions
Can I use the example per-person rate?
Replace it with your household meter or use records when possible.
Why divide by 90%?
It converts usable required water to nominal capacity when some tank volume cannot be drawn.
Does this include refilling?
No. It estimates reserve before replenishment.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.