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Calculate how much concrete you need in cubic metres for slabs, footings, columns, and post holes. Includes pre-mix bag estimates and ready-mix truck quantities.
Slab / Pad dimensions
Add 10% wastage
Recommended — accounts for spillage, uneven ground, and over-excavation
Result
Total volume
Including 10% wastage
0.000 m³
Volume (no wastage)
Exact calculated volume
0.000 m³
Wastage amount
10% extra
0.000 m³
Pre-mix bags (20 kg)
~0.009 m³ per bag
0 bags
Pre-mix bags (25 kg)
~0.012 m³ per bag
0 bags
Est. pre-mix cost (20 kg)
0 bags × $8.50
$0.00
Ready-mix equivalent
Minimum order usually 0.2 m³
0.00 m³
Est. ready-mix cost
~$250 per m³ delivered
$0.00
Truck loads
Standard truck = 5 m³
0 trucks
Common Projects — Quick Reference
| Project | Typical Size | Depth | Approx m³ |
|---|---|---|---|
| Garden path | 5m × 0.6m | 75mm | 0.23 |
| Single garage slab | 3m × 6m | 100mm | 1.80 |
| Double garage slab | 6m × 6m | 100mm | 3.60 |
| Patio / outdoor area | 4m × 4m | 100mm | 1.60 |
| Fence post hole | 200mm dia × 600mm | — | 0.019 each |
| Deck pier | 300mm dia × 600mm | — | 0.042 each |
| Strip footing (house) | 20m × 300mm | 300mm | 1.80 |
Click any row to load it into the calculator.
Concrete strength grades
Residential slabs, paths, light footings
Driveways, structural slabs, footings
Commercial, heavy-duty, suspended slabs
Industrial, high-performance structural
Mixing tips
Australian standards
The formulas
Slab / Footing:
V = L × W × D
Column / Post Hole:
V = π × r² × h × n
With wastage:
Total = V × 1.10
To calculate how much concrete you need, measure the length, width, and depth of the area in metres. Multiply them together to get the volume in cubic metres (m³). For circular shapes like columns and post holes, use the formula π × r² × depth. Always add 10% for wastage to account for spillage, uneven surfaces, and over-excavation. A standard 20 kg pre-mix bag covers approximately 0.009 m³, so you'll need around 111 bags per cubic metre.
Pre-mix bags (available at Bunnings, Mitre 10, and hardware stores) are ideal for small jobs under 0.25 m³ — things like fence posts, small pads, and repairs. For anything larger, ready-mix concrete delivered by truck is more cost-effective and gives a more consistent result. Ready-mix typically costs $200–$300 per m³ delivered, with minimum orders around 0.2–0.5 m³ depending on the supplier. For structural work like house slabs and footings, ready-mix is strongly recommended as it meets AS 3600 requirements for strength and consistency.