Estimate the volume of concrete and number of bags needed for a slab.
Volume = length × width × depth. Bags needed = volume ÷ yield per bag (a standard 25kg/50lb bag yields roughly 0.017 m³/0.6 ft³, varies by mix).
This calculator gives you a wet volume of concrete, but cement, sand and aggregate are bought by dry volume, and dry volume is roughly 1.54 times the wet volume you're pouring — that 54% isn't a rounding buffer, it's the air gap between loose aggregate particles that compaction removes. Skip that factor and you will fall short on-site, which is the single most common concrete-quantity mistake on small Indian builds.
For the most common residential grade, M20 (a 1:1.5:3 cement:sand:aggregate ratio), one cubic metre of finished concrete needs about 403 kg of cement — just over 8 bags of 50 kg each — plus roughly 0.42 m³ of sand and 0.84 m³ of coarse aggregate. M15 (1:2:4) uses less cement per cubic metre, M25 (1:1:2) uses more; if your site engineer has specified a grade, use that ratio rather than defaulting to M20. Always add 5–10% extra to whatever the calculator shows for spillage, over-excavation and site wastage — contractors who order the bare calculated number are the ones who run out mid-pour.
Cement, sand and aggregate are granular and packed loosely before mixing, so they contain air voids between particles. Once mixed with water and compacted, those voids collapse and the material settles into a denser wet volume. The standard 1.54 dry-to-wet factor accounts for this, and it's why cement bag counts based on raw m³ figures (without the 1.54 multiplier) always come up short.
For M20 (1:1.5:3), a rule-of-thumb figure is about 8 bags of 50 kg cement per cubic metre of finished concrete. That number changes with grade — M25 needs more cement per m³, M15 needs less — so confirm the mix ratio your engineer or municipal approval specifies before ordering.
5–10% above the calculated figure is standard practice, covering spillage during mixing and transport, minor over-excavation of footings, and formwork that isn't perfectly true. Larger pours with more handling steps (ready-mix delivered and pumped, for instance) tend to need less extra than small hand-mixed batches.
Yes — RMC suppliers bill by the cubic metre of wet, delivered concrete, so you order the wet volume directly rather than back-calculating from bags. The 1.54 dry-factor only matters if you're buying loose cement, sand and aggregate for site mixing; for RMC, just add your own 5% buffer for the concrete that stays in the drum, chute and formwork gaps.