Why "dry volume" is bigger than the wet volume you're pouring
When you calculate how much concrete you need, you first work out the wet (final, mixed and poured) volume — length × width × depth. But the dry ingredients (cement powder, sand, and aggregate) take up more combined space before mixing than the finished wet concrete does, because water and mixing close up the voids between the dry particles. The standard rule of thumb used across Indian civil-engineering practice is that dry volume = wet volume × 1.54.
Splitting dry volume by your mix ratio
Once you have the dry volume, it's divided among cement, sand, and aggregate according to your mix ratio — expressed as cement:sand:aggregate. Common nominal mixes are M15 (1:2:4, a leaner, lower-strength mix), M20 (1:1.5:3, a common general-purpose mix), and M25 (1:1:2, a richer, higher-strength mix). Each part of the ratio gets that fraction of the total dry volume — for M20, cement gets 1 out of 5.5 total parts, sand gets 1.5 out of 5.5, and aggregate gets 3 out of 5.5.
Cement volume converts to bags using the standard figure of about 0.0347 m³ per 50 kg bag (derived from cement's loose bulk density, roughly 1,440 kg/m³).
A worked example — M15 vs M20 for the same slab
For a 5m × 4m slab, 0.1m thick (2 m³ wet volume, 3.08 m³ dry volume after the 1.54 factor): using M20 (1:1.5:3) needs about 16.1 cement bags, 0.84 m³ of sand, and 1.68 m³ of aggregate. Using the leaner M15 (1:2:4) mix for the exact same slab instead needs only about 12.7 cement bags, but more sand (0.88 m³) and aggregate (1.76 m³) — the total dry volume doesn't change, but a leaner mix shifts more of it toward sand and aggregate and less toward (more expensive) cement.
Choosing the right mix ratio
The mix ratio isn't just about cost — it determines the concrete's compressive strength. M15 is commonly used for non-structural work like flooring base or pathways; M20 is a widely used general-purpose structural mix for slabs, beams, and columns in residential construction; M25 and richer mixes go into structures needing higher load-bearing strength. Always follow your structural engineer's specified grade for any load-bearing work — this calculator estimates material quantities for a given mix, it doesn't tell you which mix your specific structure needs.
Practical estimating tips
Add a buffer (commonly 5-10%) on top of the calculated quantities for on-site wastage, spillage, and minor measurement variance — running short mid-pour is far more disruptive than having a small surplus. Also confirm your supplier's actual cement bag weight and bulk density figures for sand and aggregate, since these vary somewhat by source and moisture content.