How it is calculated
- Wet volume = length × width × thickness (or π r² h for a round column).
- Dry volume = wet volume × 1.54 (for voids and shrinkage).
- For 1:2:4 (total 7 parts): cement = dry volume × 1/7, sand × 2/7, gravel × 4/7.
- Cement density ≈ 1,440 kg/m³; a 50 kg bag ≈ 0.0347 m³.
| Ratio | Typical use |
|---|---|
| 1:1.5:3 | Columns and beams (≈ M20) |
| 1:2:4 | Ordinary slabs and footings (≈ M15) |
| 1:3:6 | Blinding and light floors |
Worked example
A 5 × 4 m slab, 0.15 m thick, at 1:2:4: volume 3 m³, dry 4.62 m³, cement 0.66 m³ ≈ 950 kg = 20 bags, sand 1.32 m³, gravel 2.64 m³.
Frequently asked questions
How much water?
A water–cement ratio of about 0.45–0.55 by weight, roughly 25 litres per bag.
Does this apply to ready-mix?
For ready-mix, order the wet volume in m³ plus 5–10%.
Does it replace an engineer?
No — structural mixes and reinforcement must be set by an engineer.