How this calculator works
A concrete slab's material requirement comes down to one number: volume. Multiply length × width × depth in the same units, and you get cubic meters (or cubic yards). From there, standard mix ratios tell you exactly how much cement, sand and gravel that volume needs.
We use the 1:2:3 nominal mix — one part cement, two parts sand, three parts gravel by volume. That's the most common ratio for residential slabs, driveways, garages and patios. For each cubic meter of concrete, the typical breakdown is:
- ~320 kg cement (7 bags at 50 kg, rounded up)
- ~0.6 m³ sand (concrete-grade sharp sand)
- ~0.9 m³ gravel (20mm crushed stone aggregate)
We round cement bags up to the next whole bag because suppliers don't sell partial bags. Sand and gravel are continuous, so we keep two decimals for accuracy. If your project is large enough to justify ready-mix delivery (usually anything over 4–5 m³), call your supplier with the volume number and they'll quote you delivered.
Common slab sizes and their requirements
| Use case | Size (m) | Volume | Cement bags |
|---|---|---|---|
| Garden shed | 3 × 3 × 0.10 | 0.9 m³ | 6 bags |
| Single-car driveway | 5 × 3 × 0.10 | 1.5 m³ | 10 bags |
| Patio | 4 × 4 × 0.10 | 1.6 m³ | 11 bags |
| Two-car garage | 6 × 6 × 0.15 | 5.4 m³ | 35 bags |
| Workshop foundation | 8 × 6 × 0.15 | 7.2 m³ | 47 bags |
Numbers are approximate. Always order 5–10% extra to account for spillage and consolidation.