How to calculate concrete: cement, sand and aggregate for a slab
Ordering concrete materials by eye is how a job ends up half a day short of cement on a Sunday. The calculation is four steps, and the one people miss is the second.
Step 1: Work out the wet volume
This is the space the finished concrete will fill: length × width × thickness, in consistent units.
Take a slab 10 feet × 10 feet, 4 inches thick. Convert to metres first — mixing feet and inches in the same multiplication is the most common error here.
10 ft = 3.048 m
4 in = 0.1016 m
Wet volume = 3.048 × 3.048 × 0.1016 = 0.944 m³
Step 2: Multiply by 1.54 for dry volume
Here is the step that catches people out. You buy cement, sand and aggregate dry, and dry materials contain a great deal of air. Once mixed with water, the sand fills the voids between the aggregate and the cement fills the voids between the sand, and the whole thing shrinks by roughly a third.
To end up with 0.944 m³ of set concrete you need about 54% more dry material:
Dry volume = 0.944 × 1.54 = 1.454 m³
Skip this and you will order two thirds of what the job needs. The 1.54 factor is standard practice; some engineers use 1.52 or 1.57 depending on the materials.
Step 3: Split by the mix ratio
Nominal mixes are given as cement : sand : aggregate, by volume.
| Grade | Ratio | Typical use |
|---|---|---|
| M5 | 1 : 5 : 10 | Levelling course |
| M7.5 | 1 : 4 : 8 | Non-structural fill |
| M10 | 1 : 3 : 6 | Plain concrete, kerbs |
| M15 | 1 : 2 : 4 | Footpaths, flooring base |
| M20 | 1 : 1.5 : 3 | Domestic slabs, columns, beams |
| M25 | 1 : 1 : 2 | Structural work |
M20 is the everyday domestic slab mix. Its ratio adds up to 5.5 parts, so:
Cement = 1.454 × (1 ÷ 5.5) = 0.264 m³
Sand = 1.454 × (1.5 ÷ 5.5) = 0.396 m³
Aggregate = 1.454 × (3 ÷ 5.5) = 0.793 m³
Anything M25 and above should be a design mix specified by an engineer, not a nominal ratio off a table.
Step 4: Convert cement to bags
A 50 kg bag of cement occupies about 0.0347 m³.
Cement bags = 0.264 ÷ 0.0347 = 7.6 → 8 bags
Always round up. Sand and aggregate are usually ordered in cubic feet in India:
Sand = 0.396 m³ × 35.3147 = 14 cft
Aggregate = 0.793 m³ × 35.3147 = 28 cft
The answer
For a 10 × 10 foot slab at 4 inches thick, M20:
Cement 8 bags (about 381 kg)
Sand 14 cft
Aggregate 28 cft
Total weight about 2,265 kg
Add wastage
Every calculation above assumes nothing is spilled, over-ordered at the edges or left in the mixer. In practice add 5–10% — closer to 10% for an awkward pour, a hand mix or an inexperienced crew. Running short mid-pour is far more expensive than a leftover bag, because a cold joint in a slab is a permanent weakness.
Water, and why less is better
The water-cement ratio is typically 0.45 to 0.55 by weight — around 20 to 25 litres per 50 kg bag. Extra water makes concrete easier to pour and significantly weaker, and it is the single most common reason a domestic slab underperforms. If it is too stiff to work, the answer is a plasticiser or a wetter mix design, not a bucket more water.
What this does not cover
- Reinforcement. Steel is calculated separately from the structural drawing.
- Formwork. Shuttering area, not volume.
- Ready-mix. If you are buying RMC, you order the wet volume — 0.944 m³ here, plus wastage — and the supplier handles the proportions.
Do it for your own dimensions
The Concrete Calculator takes the dimensions and the grade and gives cement bags, sand and aggregate with the wastage allowance included. For the rest of a build, there are matching calculators for bricks, cement mortar, tiles and paint.