Free Cement Sand Aggregate calculator with clear step-by-step results.
Dry ingredients occupy roughly 54% more volume than the concrete they produce, because water fills the voids between particles. The wet volume is grossed up by that dry factor, split by the nominal mix ratio, and cement volume is converted to bags using a bulk density of 1440 kg per cubic metre.
Dry volume
Dry volume = wet volume x 1.54
Cement bags
Bags = dry volume x (cement part / sum of parts) x 1440 / 50
Nominal mixes are for non-critical work only. Structural concrete should be specified by design mix with tested materials and verified cube strengths.
The factor covers both bulking of dry sand and the voids in coarse aggregate. Values between 1.52 and 1.57 are all in common use; 1.54 is the most widely quoted.
It will place more easily and be markedly weaker. Above about 0.55 water to cement, strength and durability fall away quickly - use a plasticiser instead.