A method for turning occupancy and a per-person allowance into a cistern capacity, with the arithmetic shown.
This example shows how to estimate the size of a cold water storage cistern. The numbers are there to show the arithmetic: take the allowance per person from your own design guidance.
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 — check usable capacity. The water stops below the brim, and some may sit below the outlet where it cannot be drawn off. So for the example’s 400-litre reserve, check from the product data and the installation that at least 400 litres can actually be drawn off: the nominal size does not tell you.
Step 3 — choose the next size up. Pick a standard cistern providing at least 400 litres of usable capacity, for example a unit with 450 litres available for draw-off. Choose by usable volume, not nominal size.
Takeaway. Multiply the occupants by the allowance, then choose a cistern with at least that much usable capacity. In a real design, also check demand, refill rate and turnover, so the reserve is big enough without leaving too much water standing.
Working out the stored volume from its dimensions
A cistern's actual stored volume is length times width times the depth of water, not the depth of the cistern itself: measure down only to the shut-off water line, since the valve stops filling before the brim.
For example, a cistern 1.0 m long and 0.6 m wide with water 0.4 m deep holds 0.24 cubic metres, or 240 litres. This is the stored volume at that water line. The usable draw-off can be less if water remains below the outlet or a required minimum operating level.