Step one: total wet volume
For 6 identical columns, each 0.3m × 0.3m × 3m: 0.3 × 0.3 × 3 × 6 = 1.62 cubic meters of wet (as-poured) concrete. For 6 footings, each 1.2m × 1.2m × 0.5m: 1.2 × 1.2 × 0.5 × 6 = 4.32 cubic meters.
Step two: converting to dry volume
Dry ingredients (cement, sand, aggregate) take up more space loose than they do once mixed and set, so the wet volume is scaled up by a wastage allowance (5% in both examples here) and then by the standard site-estimation factor of 1.54, before splitting into materials.
Step three: splitting by mix ratio
An M20 mix (ratio 1:1.5:3, meaning 1 part cement to 1.5 parts sand to 3 parts aggregate, 5.5 parts total) applied to the column example gives 13.7 cement bags (50kg each), 0.714 cubic meters of sand, and 1.429 cubic meters of aggregate. The same M20 mix applied to the larger footing volume gives 36.6 cement bags, 1.905 cubic meters of sand, and 3.810 cubic meters of aggregate — proportionally larger, matching the bigger wet volume.
Why columns and footings use the same three-step process
Despite being different structural elements, a column and a footing are still both just rectangular volumes multiplied by a count — the only difference is which dimensions get multiplied together first. Once wet volume is known, the dry-volume and mix-ratio steps that follow are identical for either one.