Concrete
Concrete Calculator
Work out cubic metres, 20kg bags and cost for slabs, footings, post holes and stairs.
Estimates are a guide only. Confirm quantities with your concrete supplier before ordering, and use an engineer for structural slabs.
How this calculator works
The calculator multiplies your dimensions to get a volume in cubic metres. For a slab that is length by width by thickness, for footings it is length by width by depth, and for post holes it uses the circle area of each hole multiplied by depth and the number of holes.
A 10 percent waste allowance is added by default. Concrete is lost to uneven ground, spillage and over excavation, and running short mid pour is expensive, so ordering a little extra is standard practice. You can switch the allowance off if you have already accounted for waste.
Bag counts assume a 20kg bag of premixed concrete yields about 0.009 cubic metres once mixed. Costs use typical Australian prices: premixed concrete delivered at $270 to $330 per cubic metre and bags at $9 to $13 each.
| Application | Typical thickness | Notes |
|---|---|---|
| Garden paths and patios | 75mm to 100mm | Light foot traffic only |
| Shed slabs | 100mm | Check your shed manufacturer specs |
| Driveways (cars) | 100mm to 125mm | With reinforcing mesh |
| Driveways (heavy vehicles) | 150mm to 200mm | Engineer design recommended |
| House slabs | 100mm plus edge beams | Always engineer designed |
Typical Australian practice. Confirm thickness with your builder or engineer for structural work.
| Supply type | Typical price | Best for |
|---|---|---|
| Premixed, delivered | $270 to $330 per m³ | Jobs over about 0.3m³ |
| 20kg bags | $9 to $13 per bag | Small jobs, repairs, post holes |
| Minimum load fee | Often applies under 3m³ | Ask your supplier |
Prices vary by region, mix strength and access. Get local quotes for accurate pricing.
Frequently asked questions
How many 20kg bags of concrete make 1 cubic metre?
Around 110 bags. One 20kg bag yields roughly 0.009 cubic metres of mixed concrete, so mixing a full cubic metre by hand is a big job. For anything over about 0.3 cubic metres, premixed concrete is usually cheaper and far less work.
What thickness should a concrete slab be?
For paths and patios 75mm to 100mm is typical. Driveways for cars are usually 100mm to 125mm with reinforcing mesh, and heavy vehicle areas need 150mm or more. House slabs are always engineer designed.
Why add 10 percent for waste?
Ground is never perfectly level, some concrete stays in the truck or barrow, and formwork can flex. Running short mid pour means a cold joint and a second delivery fee, so ordering about 10 percent extra is standard industry practice.
How much does concrete cost in Australia?
As at July 2026, premixed concrete typically costs $270 to $330 per cubic metre delivered, depending on region, strength grade and access. Decorative finishes, pumping and small load fees add to that.
Should I order premixed concrete or mix bags?
Bags suit small jobs like a few post holes or a repair, roughly 0.3 cubic metres or less. Beyond that, premixed wins on price, consistency and effort. Enter your job above and the calculator compares both costs for you.
How is concrete volume calculated?
Multiply length by width by thickness in metres to get cubic metres. For example a 6m by 4m slab at 100mm thick is 6 by 4 by 0.1, which is 2.4 cubic metres, or 2.64 cubic metres with 10 percent waste.
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