M20 is the most commonly used concrete grade in residential construction. This article shows you exactly how to calculate the quantity of cement bags, sand, and coarse aggregate needed — with a full worked example.
What is M20 Concrete?
M20 concrete has a mix ratio of 1:1.5:3 (Cement : Sand : Coarse Aggregate) and a characteristic compressive strength of 20 N/mm² at 28 days. It is widely used for slabs, beams, columns, and footings in residential and light commercial buildings.
| Grade | Mix Ratio (C:S:A) | Compressive Strength | Common Use |
|---|---|---|---|
| M15 | 1:2:4 | 15 N/mm² | PCC, non-structural |
| M20 | 1:1.5:3 | 20 N/mm² | Slabs, beams, columns |
| M25 | 1:1:2 | 25 N/mm² | Heavy structures, bridges |
Step-by-Step Calculation
Given Data (Example)
- Volume of concrete required = 1 m³
- Mix ratio = 1:1.5:3 (M20)
- Dry volume factor = 1.54 (to account for voids and compaction)
Note: We multiply wet volume by 1.54 to get the dry volume of materials required, as concrete shrinks after mixing.
Step 1 — Find Dry Volume
Formula: Dry Volume = Wet Volume × 1.54 Dry Volume = 1 × 1.54 = 1.54 m³
Step 2 — Total Parts in Mix Ratio
Total Parts = 1 + 1.5 + 3 = 5.5 parts
Step 3 — Volume of Cement
Formula: Volume of Cement = (Cement Parts / Total Parts) × Dry Volume = (1 / 5.5) × 1.54 = 0.28 m³ No. of Cement Bags = Volume of Cement / Volume of 1 bag = 0.28 / 0.0347 = 8.07 ≈ 8 bags
Volume of 1 bag of cement = 50 kg / 1440 kg/m³ = 0.0347 m³
Step 4 — Volume of Sand
Volume of Sand = (1.5 / 5.5) × 1.54 = 0.42 m³ = 0.42 × 35.32 = 14.83 CFT
Step 5 — Volume of Coarse Aggregate
Volume of Aggregate = (3 / 5.5) × 1.54 = 0.84 m³ = 0.84 × 35.32 = 29.67 CFT
Summary Table
| Material | Quantity (m³) | Quantity (CFT) | In Practical Units |
|---|---|---|---|
| Cement | 0.28 m³ | — | 8 bags (50 kg each) |
| Sand (FA) | 0.42 m³ | 14.83 CFT | ~630 kg |
| Coarse Aggregate | 0.84 m³ | 29.67 CFT | ~1260 kg |
Important: These quantities are for 1 m³ of wet concrete. For a slab or beam, first calculate the total volume of concrete needed, then multiply.
Quick Reference: Cement Bags Required per m³
| Concrete Grade | Mix Ratio | Cement Bags per m³ |
|---|---|---|
| M10 | 1:3:6 | 4.4 bags |
| M15 | 1:2:4 | 6.3 bags |
| M20 | 1:1.5:3 | 8.0 bags |
| M25 | 1:1:2 | 10.8 bags |
Conclusion
For 1 m³ of M20 concrete, you need approximately 8 bags of cement, 0.42 m³ of sand, and 0.84 m³ of coarse aggregate. Always account for wastage by adding 5–10% extra when ordering materials on site.
