How to calculate concrete for a slab
A slab is a simple rectangular pour, so the wet concrete volume is:
Volume (m³) = Length (m) × Width (m) × Thickness (m)
Keep the units consistent — a 150 mm slab is 0.15 m thick. The calculator above lets you enter the thickness in millimetres directly. To get the materials, the wet volume is multiplied by 1.54 for the dry volume, then split by the mix ratio (cement : sand : aggregate).
Concrete needed for common slab sizes (at 150 mm)
| Slab size | Volume (150 mm) | Cement (M20) |
|---|---|---|
| 3 × 3 m | 1.35 m³ | ≈ 11 bags |
| 4 × 4 m | 2.40 m³ | ≈ 19 bags |
| 5 × 4 m | 3.00 m³ | ≈ 24 bags |
| 6 × 5 m | 4.50 m³ | ≈ 36 bags |
Cement based on M20 (1:1.5:3), 50 kg bags and a 1.54 dry-volume factor. Add a wastage allowance and confirm with your mix design.