Concrete slab calculator – m³, cubic yards and bags
How much concrete for a slab: the formula in metric and imperial, how many 25 kg / 40–80 lb premix bags, waste allowance and worked examples.
Volume = length × width × thickness · order +5–10 % for waste and uneven base
Worked examples
| Slab | Volume | Order |
|---|---|---|
| 5 × 4 m, 10 cm | 2.0 m³ | 2.1–2.2 m³ |
| 20 × 20 ft, 4 in | 133 cu ft = 4.9 cu yd | 5.2–5.5 cu yd |
| 3 × 3 m shed base, 15 cm | 1.35 m³ | 1.4–1.5 m³ |
Bags of premixed concrete
| Bag | Yield (approx.) | Bags per m³ / per cu yd |
|---|---|---|
| 25 kg | ≈ 0.011 m³ | ≈ 90–95 per m³ |
| 40 lb | 0.30 cu ft | 90 per cu yd |
| 60 lb | 0.45 cu ft | 60 per cu yd |
| 80 lb | 0.60 cu ft | 45 per cu yd |
Above about 1 m³ (1.3 cu yd) ready-mix from a truck is usually cheaper and gives a better slab than mixing bags. Conversions: 1 m³ = 1.308 cu yd = 35.31 cu ft.
Site tips
- Compact the sub-base and use a damp-proof membrane under ground-bearing slabs.
- Place reinforcement (mesh or rebar) on chairs/spacers so it sits in the slab, not on the ground.
- Keep the surface wet or covered for at least 7 days; do not pour below about 5 °C without precautions.
- Slab thickness and reinforcement for anything structural must be designed by a licensed engineer.
AI Onin is an AI assistant for construction: material quantities, cost estimates in your currency, bill of quantities checks and site photo analysis. Values are typical and approximate – follow the manufacturer's data sheet, your local code and your engineer.