Concrete Calculator Australia
Calculate how much concrete you need for a slab, footing or other rectangular pour.
Your result
Enter your dimensions above to see how much concrete you need.
It does not design a structurally adequate slab, footing or any other structural element. Slab thickness, reinforcement, concrete strength and ground preparation depend on your project, site conditions and the applicable Australian Standards and council requirements. Check final quantities and specifications with your supplier, builder or engineer.
How the concrete calculation works
Concrete is sold by volume, so working out how much you need is just multiplying three measurements together:
Volume (m³) = length (m) × width (m) × depth (m)
The only trap is units. Slab thickness is almost always quoted in millimetres while length and width are in metres, so the thickness has to be converted before you multiply. Divide millimetres by 1,000: 100 mm = 0.1 m.
Worked example
A slab 5 metres long, 4 metres wide and 100 mm thick:
- Convert the thickness: 100 mm ÷ 1,000 = 0.1 m
- Multiply: 5 × 4 × 0.1 = 2.00 m³
- Add a 10% allowance: 2.00 × 1.1 = 2.20 m³
The calculator above does all of this for you, including the unit conversion, whichever units you choose.
Why add a waste allowance?
The volume you calculate is the volume of a perfect box. Real pours are not perfect boxes. Concrete spreads slightly beyond the formwork, sub-grade is never perfectly level, some is left in the barrow or the chute, and a little is lost to spillage.
A 10% allowance is a common starting point and is the default here. You might use less on a small, tightly formed pour with good ground preparation, and more on uneven ground or where the excavation is rough.
Running out mid-pour is a far worse outcome than ordering slightly too much — a cold joint where fresh concrete meets concrete that has already started to set is a genuine weakness, and a second delivery costs more than the extra you would have ordered. How much allowance is right varies by job and by supplier, so treat the figure as a starting point and talk it through with whoever is supplying the concrete.
What you can actually order
Suppliers do not sell arbitrary amounts. Ready-mix is typically sold in 0.2 m³ increments, and most suppliers apply a minimum load or a minimum charge — so a very small pour may cost the same whether you order 0.4 m³ or 1 m³. The calculator shows a suggested order quantity rounded up to the next increment for exactly this reason. Confirm your supplier's increments and minimum before ordering.
Common slab thicknesses
These are general examples of thicknesses often seen in domestic work, provided to help you use the calculator. They are not a specification and not engineering advice.
| Application | Often around |
|---|---|
| Garden path or stepping area | 75–100 mm |
| Shed base or small slab | 100 mm |
| Domestic driveway (cars) | 100–125 mm |
| Driveway with heavier vehicle access | 150 mm+ |
What your slab actually needs depends on the loads it will carry, your soil classification, reinforcement, concrete strength class and the relevant Australian Standards. Reactive clay sites in particular have requirements that no calculator can determine. If the slab is structural or supports a building, it needs to be specified by someone qualified to do so.
Bagged concrete
For small jobs, premix bags are usually easier than arranging a delivery. To estimate bags, you need the yield of the product — the volume of wet concrete one bag makes once it is mixed.
This is where most bag estimates go wrong. Yield is not determined by the weight of the bag. Two different 20 kg products can produce noticeably different volumes depending on the mix. The yield printed on the bag is the only figure that counts, and our calculator lets you enter it directly.
As a rough guide, a 20 kg bag of general-purpose concrete mix commonly yields somewhere around 9 litres, which works out to roughly 110 bags per cubic metre. That number climbs quickly: even a modest 2 m³ slab would be well over 200 bags, which is a full day of mixing for a crew. Somewhere around half a cubic metre, ready-mix delivery usually becomes both cheaper and far less painful.
Use the concrete bag calculator if bags are what you are after.
Cement or concrete — which do you need?
These get used interchangeably, but they are not the same thing. Cement is a powder. It is one ingredient of concrete, the binder that holds everything together. Concrete is cement plus sand, aggregate (stone) and water.
If you are pouring a slab, footing, path or driveway, you need concrete, and this calculator is the right one — even if you searched for a "cement calculator". You would only calculate cement on its own if you were batching a mix from separate materials or working out mortar.
More worked examples
Small shed base — 3 m × 3 m × 100 mm
3 × 3 × 0.1 = 0.90 m³. With a 10% allowance, 0.99 m³. Rounded to the next 0.2 m³ increment, you would order 1.0 m³.
Garden path — 10 m × 0.9 m × 75 mm
10 × 0.9 × 0.075 = 0.675 m³. With a 10% allowance, 0.74 m³ — order 0.8 m³. This is right on the boundary where bags stop being sensible: at roughly 9 L per bag that is about 83 bags.
Single driveway — 12 m × 3 m × 125 mm
12 × 3 × 0.125 = 4.50 m³. With a 10% allowance, 4.95 m³ — order 5.0 m³. Comfortably a ready-mix job.
Frequently asked questions
How much concrete do I need for a 5 × 4 metre slab?
At 100 mm thick, 2.00 m³, or 2.20 m³ with a 10% waste allowance. At 150 mm thick it is 3.00 m³, or 3.30 m³ with the allowance.
How many 20 kg bags of concrete make a cubic metre?
Roughly 110, if the bag yields about 9 litres. Check the yield printed on your product — it varies between brands and mixes, and that figure is the one to use.
How do I convert millimetres to metres for the depth?
Divide by 1,000. So 100 mm = 0.1 m, 125 mm = 0.125 m, and 150 mm = 0.15 m. The calculator handles this automatically when you select mm.
Should I order the exact volume I calculated?
Generally no. Add an allowance for spillage and uneven sub-grade, then round up to your supplier's selling increment. Running short mid-pour creates a cold joint and costs more to fix than the extra concrete would have cost.
Is a cement calculator the same as a concrete calculator?
For a slab or footing, yes — people search for both but they almost always mean concrete volume. Cement is just one ingredient of concrete.
Does this calculator tell me how thick my slab should be?
No. It calculates volume from the dimensions you enter. Thickness, reinforcement and concrete strength depend on the loads, your soil and the applicable standards, and should be specified by your builder or engineer.