Key takeaways
- Dry volume = wet volume × 1.54 — always start here.
- Split the dry volume by the mix ratio (e.g. M20 = 1 : 1.5 : 3, total 5.5 parts).
- Cement bags = cement volume ÷ 0.0347 m³ (one 50 kg bag).
- M20 needs roughly 8 bags of cement per m³ of finished concrete.
The quick answer
To find materials for concrete, take the wet (finished) volume you need, multiply by 1.54 to get the dry volume, then divide that dry volume between cement, sand and aggregate using the mix ratio. Convert the cement portion to 50 kg bags by dividing by 0.0347 m³ (the volume of one bag). For a common M20 mix that works out to about 8 bags of cement, 0.42 m³ of sand and 0.84 m³ of aggregate per cubic metre of concrete.
Why the 1.54 factor exists
When you mix dry cement, sand and aggregate with water and compact it, the total shrinks: water fills the voids between the sand grains and the aggregate, and the paste settles into the gaps. To end up with 1 m³ of solid concrete you have to start with roughly 1.54 m³ of loose dry materials. This dry-volume factor is usually taken as 1.54 (some references use 1.52–1.57). Everything else is simple proportion.
One 50 kg bag of cement has a loose volume of about 0.0347 m³ (and equals roughly 1.226 CFT), which is the conversion that turns volume into bags you can order.
Nominal mix ratios
Nominal mixes are the by-volume recipes used on most ordinary sites without lab design. The "M" number is the 28-day characteristic strength in N/mm².
| Grade | Ratio (C : S : A) | Total parts | Typical use |
|---|---|---|---|
| M5 | 1 : 5 : 10 | 16 | Lean / blinding layer |
| M7.5 | 1 : 4 : 8 | 13 | Levelling, non-structural |
| M10 | 1 : 3 : 6 | 10 | PCC, flooring base |
| M15 | 1 : 2 : 4 | 7 | PCC, light footings |
| M20 | 1 : 1.5 : 3 | 5.5 | General RCC slabs, beams |
| M25 | 1 : 1 : 2 | 4 | Columns, heavier loads |
A worked example: an M20 slab
Say you are casting a slab with a finished (wet) volume of 5 m³ in M20 (1 : 1.5 : 3, total 5.5 parts). First the dry volume: 5 × 1.54 = 7.7 m³. Now split it:
- Cement: 7.7 × (1 ÷ 5.5) = 1.40 m³ → 1.40 ÷ 0.0347 = ≈ 40 bags (2,000 kg).
- Sand: 7.7 × (1.5 ÷ 5.5) = 2.10 m³ (≈ 74 CFT, or about 3.3 tonne at 1,550 kg/m³).
- Aggregate: 7.7 × (3 ÷ 5.5) = 4.20 m³ (≈ 148 CFT, or about 6.1 tonne at 1,450 kg/m³).
That checks out against the "8 bags per m³" rule of thumb (40 ÷ 5 = 8). The concrete calculator runs all of this instantly for any volume and grade, and reports sand and aggregate in m³, CFT and tonnes.
For a reinforced slab, add steel
The mix above covers only the concrete. A reinforced (RCC) slab also needs reinforcement steel — typically around 80 kg per cubic metre of slab. The RCC slab calculator combines the concrete quantities with a steel estimate so you can plan both in one place.
Frequently asked questions
Why multiply wet volume by 1.54?
Dry materials compact and their voids fill when mixed with water, so you need ~54% more loose material than the finished volume. 1.54 is the common dry-volume factor (some use 1.52–1.57).
How many cement bags per cubic metre of M20?
About 8 bags per m³: dry volume 1.54 m³ × (1 ÷ 5.5) ÷ 0.0347 ≈ 8.06 bags.
What is the difference between M15, M20 and M25?
The number is 28-day strength in N/mm². M15 (1:2:4) is for levelling, M20 (1:1.5:3) is the common structural grade, and M25 (1:1:2) is richer for columns.
Sources: the dry-volume factor (≈1.54), nominal mix ratios (M5 1:5:10 … M25 1:1:2) and the 50 kg cement bag volume (0.0347 m³ ≈ 1.226 CFT) follow standard trade practice as taught in standard Pakistani civil-engineering references. Bulk densities used: sand ≈ 1,550 kg/m³, coarse aggregate ≈ 1,450 kg/m³.
Last reviewed 2026-06-14