Cement calculator
A 1 : 2 : 4 mix means one part cement, two parts sand, four parts aggregate, measured by volume rather than weight. Seven parts go in and about five come out, because the sand fills the gaps between the stones and the cement paste fills the gaps between the sand. That shrinkage is why a bucket count never quite matches the hole.
Cement is the binder, not the whole mix. A standard 1 : 2 : 4 concrete uses one part cement to two of sand and four of aggregate by volume, and seven parts of dry material yield roughly five parts of finished concrete because the finer materials fill the voids in the coarser ones.
How to size a cement order
The mix ratio you want depends on the job rather than on preference. A 1 : 2 : 4 is general-purpose concrete for slabs and footings. A 1 : 3 : 6 is a weaker fill for backfilling and bedding, and a 1 : 1.5 : 3 is a stronger structural mix. For mortar rather than concrete you drop the aggregate entirely: 1 : 4 cement to sharp sand for bricklaying, softer and richer for pointing. Water is the one ingredient never listed in the ratio, and the one most likely to ruin the batch — more water makes it easier to work and weaker forever.
Questions
For a 1 : 2 : 4 concrete, roughly 13 bags of 25 kg per cubic metre of finished concrete, plus about 0.5 m³ of sand and 0.75 m³ of aggregate.