Concrete Calculator: Cubic Yards and Cubic Metres for Slabs and Footings
Concrete is ordered by volume, so the calculation is length × width × thickness converted into cubic yards or cubic metres. Getting it right matters more than with most materials: a short pour has to be finished with a second batch that will not bond cleanly.
Working out the volume
Keep every dimension in the same unit before multiplying. A 20 ft × 10 ft slab at 4 inches thick is 20 × 10 × 0.333 = 66.7 cubic feet, which is 2.47 cubic yards (divide by 27). In metric, a 6 m × 3 m slab at 100 mm is 6 × 3 × 0.1 = 1.8 cubic metres.
Always order a waste allowance
- Add 5-10% for spillage, uneven subgrade and formwork spread.
- Add more on rough ground, where subgrade dips silently absorb volume.
- Ready-mix suppliers usually sell in 0.25 yd³ or 0.5 m³ increments — round up, never down.
Thickness and mix guidance
- Footpaths and patios: 100 mm / 4 in.
- Driveways for cars: 100-125 mm / 4-5 in, more for heavy vehicles.
- Garage and shed slabs: 125-150 mm / 5-6 in over compacted hardcore.
- Typical mixes are C20-C25 for domestic slabs; check local code for structural work.
Bags versus ready-mix
A 25 kg bag yields roughly 0.012 m³ and an 80 lb bag about 0.6 ft³, so anything above about 0.5 m³ (0.7 yd³) becomes impractical to mix by hand. Beyond that, ready-mix delivery is cheaper, more consistent and lets you place the whole pour before it starts to set.
Key takeaways — Concrete Calculator
- Volume = length × width × thickness, with all dimensions in the same unit.
- Divide cubic feet by 27 for cubic yards.
- Order 5-10% extra — running short mid-pour creates a cold joint.