Mix ratios are by volume, cement is sold by weight, and orders are placed in bags. Three conversions tie them together.
Core Figures
| Quantity | Value |
|---|---|
| Weight of one bag | 50 kg |
| Bulk density (for estimating) | 1,440 kg/m³ |
| Volume of one bag | 50 ÷ 1,440 = 0.0347 m³ |
| In litres | about 34.7 L (≈ 35 L box) |
| In cft | 0.0347 × 35.31 = about 1.23 cft |
| Bags per m³ | 1,440 ÷ 50 = 28.8 |
| Bags per tonne | 20 |
| Specific gravity of cement | about 3.15 |
Where the Numbers Are Used
Measuring boxes. On site, aggregates are measured in boxes sized to one bag of cement — about 35 litres, often 300 × 300 × 390 mm or similar. For a 1:1.5:3 mix: one bag, one and a half boxes of sand, three boxes of aggregate.
Mix calculations. Cement volume in a mix × 1,440 ÷ 50 = bags. For M20: 0.28 m³ × 1,440 = 403 kg = 8.06 bags per m³.
Comparing quotes. Price per tonne ÷ 20 = price per bag.
Why Bulk Density Varies
Loose cement in a heap contains air; compacted cement contains less. Figures from about 1,250 to 1,600 kg/m³ are quoted depending on compaction. Estimators in India use 1,440 kg/m³ by convention, which is why it appears in almost every calculation — use it consistently.
Checking Bag Weight
A bag should weigh 50 kg. Weigh two or three bags from each delivery on a platform scale; consistently light bags mean you are paying for cement you did not receive.
The Short Version
50 kg, 0.0347 m³, 1.23 cft, 28.8 bags per m³, 20 bags per tonne. Use 1,440 kg/m³ for every estimate, and a 35-litre box for one bag on site.
