Work out exactly how many bags of cement you need for a concreting, mortar or plastering job. Enter your area, thickness and mix ratio and the calculator applies the standard 1.54 dry-volume factor used by contractors worldwide.
For a standard 1:2:4 mix, roughly 6.3 bags (about 315 kg) of cement are needed per cubic metre of finished concrete, once the 1.54 dry-volume factor is applied.
Wet, compacted concrete volume is smaller than the loose dry materials that go into it because sand and gravel have air gaps. Multiplying by 1.54 accounts for that so you buy enough dry material.
1:1.5:3 (M20) suits load-bearing RCC work, 1:2:4 (M15) is the common general-purpose mix, and 1:3:6 or leaner mixes suit foundations and mass concrete. Check your local building code or engineer's specification for structural work.