A method for turning occupancy and a per-person allowance into a cistern capacity, with the arithmetic shown.
This example shows the method for estimating a cold water storage cistern. The numbers are chosen to demonstrate the arithmetic; the per-person allowance you use comes from your own design guidance, so treat the figure here as illustrative only.
The situation. A house has 4 occupants. For this worked example we use a storage allowance of 100 litres per person to cover normal use plus a short reserve.
Step 1 — total storage needed. Multiply occupants by the allowance: 4 people x 100 litres = 400 litres.
Step 2 — allow for usable (actual) capacity. A cistern is never full to the brim; water sits below the warning/overflow level, so the usable volume is less than the nominal size. To deliver 400 usable litres you must select a cistern whose actual capacity is at least 400 litres.
Step 3 — choose the next size up. Pick the next standard cistern with an actual capacity of 400 litres or more, for example a 450 litre actual-capacity unit, rather than one whose nominal size only just reaches 400 litres.
Takeaway. The pattern is always: occupants x allowance, then round up to the next available actual capacity. If you change the allowance you change the answer, so the method matters more than the single figure. Confirm the correct allowance and any minimum storage requirement against your current guidance before sizing a real installation.