Why use this calculator
Concrete is ordered by volume and every other part of the job is measured in area, which is where slab quantities go wrong. People measure the pad in square metres, forget that thickness is in millimetres while everything else is in metres, and order by a factor of ten out.
The second failure is ordering the exact geometric volume with no allowance: a compacted base is never perfectly flat, the screed sits proud in places, some concrete stays in the pump line and some is lost off the barrow. Running short on a pour is far worse than over-ordering, because a cold joint forms where the first load has started to set, and a cold joint in a slab is a permanent crack line.
This calculator gives you the exact volume, the volume including a waste allowance, and the rounded booking figure the batching plant will actually put on the docket, along with a bag count so you can see immediately whether bagged premix is realistic or whether you should be calling a truck.
How to use it
- Measure the slab length and width in metres at the finished edge of the formwork rather than the inside of the excavation, since the concrete fills to the form.
- Enter the thickness in millimetres straight from the engineer's detail or the shed manufacturer's slab specification, remembering that 100 mm is 0.1 metres and typing 100 into a metres field is the classic mistake.
- Set the waste allowance to suit your base: 5 per cent for a well-prepared and screeded sub-base, 10 per cent for a typical domestic job, and more if the ground is soft or the levels are rough.
- Read the yield per 20 kg bag from the bag itself, where it is printed either as litres or cubic metres of finished wet mix.
- Finally enter the delivered price per cubic metre that your supplier quoted, which should already include delivery to your postcode.
What each result means
- Slab volume
- The exact geometric volume of the rectangular prism you described, with no allowance for site losses at all.
- Volume to order with waste
- The volume plus your waste allowance, which is the realistic quantity the pour will actually consume.
- Ready-mix order rounded up
- The figure to book with the batching plant, rounded to the 0.1 cubic metre increment that trucks are batched in.
- Concrete cost per m2 of slab
- The concrete bill, including waste and the rounded booking, divided by the slab area. It lets you check the concrete share of a contractor's per square metre price.
- Equivalent 20 kg bags
- How many bags of premix would do the same job. Above roughly fifty bags, hand mixing stops being sensible.
- Approximate wet mass
- Weight of the wet concrete, which matters for truck access, suspended slabs and whether a driveway will take the delivery.
- Slab area
- The plan area, which also sets how much reinforcing mesh, vapour barrier and edge formwork you need to price.
- Estimated concrete cost
- Delivered concrete only. It excludes formwork, mesh, bar chairs, pump hire, labour, finishing and any short-load fee.
The formula
Slab area in square metres = length in metres multiplied by width in metres
Volume in cubic metres = area multiplied by thickness in metres
Order volume = volume multiplied by one plus the waste percentage divided by 100, then rounded up to the next 0.1 cubic metre because that is the increment batching plants work in
Bag count = order volume divided by the yield in cubic metres per bag, rounded up
Approximate wet mass in tonnes = order volume multiplied by 2.4, the nominal density of normal-weight concrete in tonnes per cubic metre
Cost = rounded order volume multiplied by the delivered price per cubic metre
How this number is used in practice
The decision this calculator really drives is bags against truck. A 20 kg bag of premix yields roughly nine litres of wet concrete, so a single cubic metre takes around 110 bags, which is over two tonnes of material to carry, open and mix by hand.
In practice bagged mix makes sense up to about a quarter of a cubic metre, which covers a post footing, a small landing or a short path. Beyond half a cubic metre, ready-mix is cheaper per cubic metre, vastly faster and far more consistent, because a plant batches to a specified strength while hand mixing varies bag to bag.
If the bag count here comes back above about fifty, price a truck instead, and ask about the minimum load charge before you decide.
Thickness and reinforcement are engineering decisions, not preferences. In Australia, residential slabs are designed under AS 2870, which classifies the site by how reactive the clay is and sets the slab type and thickness accordingly. A shed floor on stable sand is a different animal from a house slab on a highly reactive clay site, and neither is something to guess.
The same applies to vehicle loads: a path that carries foot traffic at 75 or 100 mm will craze and crack under a car, where a driveway usually wants 100 to 150 mm over a compacted base with mesh. Use this tool to price the design you have been given, not to choose the design.
Two site realities always add volume beyond the rectangle. Thickened edge beams and footing trenches around the perimeter can add a surprising amount, so measure and add them as a separate rectangular volume rather than assuming the waste percentage covers it. And a base that dips even 10 mm below level across a large slab adds roughly a tenth of the slab thickness in extra concrete. Screeding the base properly before the truck arrives is the cheapest concrete you will ever not buy.
Worked examples
A 6 by 4 metre shed slab at 100 mm with 10 per cent waste
2.400 cubic metres exact, 2.7 cubic metres to order, about 6.34 tonnes and 810 dollars of concrete at 300 dollars a cubic metre
A 3 by 1.2 metre garden path at 75 mm, ordered as bags
0.270 cubic metres exact and 0.3 cubic metres with waste, which is 33 bags of 20 kg premix
A 9 by 6 metre driveway at 150 mm with 5 per cent waste
8.100 cubic metres exact, 8.6 cubic metres to order and roughly 20.4 tonnes of wet concrete
When people use it
- Booking a ready-mix delivery for a shed, garage or carport slab
- Deciding whether a small pour is a bag job or worth a truck
- Checking a concreter's quoted cubic metres against your own measurements
- Budgeting the concrete line of a driveway or patio before getting quotes
- Working out delivery mass to confirm a truck can reach the pour without damaging the existing driveway
Tips
- Thickness is in millimetres and everything else is in metres. Typing 100 for a 100 mm slab into a metres field gives an answer a thousand times too large.
- Never order the exact volume. Running short creates a cold joint, which is a permanent crack line through the slab.
- Screed and compact the base to level before the truck arrives. A base 10 mm low across a big slab quietly swallows an extra load.
- Measure and add thickened edges and footing trenches separately. They are not covered by a 10 per cent waste allowance.
- Ask about the minimum load charge. Many suppliers price small deliveries as though you ordered two cubic metres anyway.
- Order slightly over and have a form ready for the surplus, such as a spare paver or a post footing, rather than sending concrete away.
What it assumes, and where it stops being accurate
- Assumes a plain rectangular slab of constant thickness. Thickened edge beams, footings, steps and rebates are not included and must be added separately.
- The 2.4 tonnes per cubic metre density is nominal for normal-weight concrete. Lightweight and high-density mixes differ, and the mass is wet mass, not cured.
- Bag yield varies between brands and between concrete, mortar and rapid-set products. Read the yield printed on the bag rather than relying on the default.
- Not accurate where the sub-base is uneven, uncompacted or waterlogged, since the slab fills every low spot and the real volume can exceed the calculation substantially.
- Does not size or specify the slab. Thickness, reinforcement and site classification are engineering decisions governed by AS 2870 in Australia and equivalent codes elsewhere.
- Price is concrete delivered only. Short-load fees, pump hire, out-of-hours surcharges, waiting time, formwork, mesh, membrane, labour and finishing are all additional.
Terms used on this page
- Ready-mix
- Concrete batched at a plant to a specified strength and delivered wet in an agitator truck.
- Cold joint
- A weak plane formed where fresh concrete is placed against concrete that has already begun to set.
- Slab yield
- The volume of finished wet concrete a bag of dry premix produces, printed on the bag in litres or cubic metres.
- Thickened edge beam
- A deeper strip of concrete around the slab perimeter that carries wall loads into the ground.
- Site classification
- A rating of how much the ground moves with moisture, which determines the slab design under AS 2870.
Questions people ask
- How many 20 kg bags make a cubic metre?
- About 110 bags at a typical yield of nine litres each, which is roughly 2.2 tonnes of dry material. Yields vary by product, so check the figure printed on the bag. At that volume, ready-mix is both cheaper and far more consistent, and hand mixing over a hundred bags is a full day of hard work.
- How thick should my slab be?
- That is a design decision, not a rule of thumb. In Australia residential slabs follow AS 2870, which sets the slab type from the site soil classification. As a rough guide, garden paths run 75 to 100 mm, shed floors 100 mm, and driveways 100 to 150 mm over a compacted base, but always follow the engineering detail for your site.
- What waste percentage should I use?
- Five per cent for a flat, well-compacted and screeded base with formwork set accurately. Ten per cent for a typical domestic job where the base is reasonable but not surveyed. Fifteen per cent or more where the ground is soft, the levels are rough, or concrete is being pumped a long distance and will be left in the line.
- Does this include reinforcement mesh and formwork?
- No. It gives concrete volume, mass and cost only. Reinforcing mesh, bar chairs, vapour barrier, formwork timber, pump or line hire, labour, and the finish you choose are all separate cost lines that usually add up to more than the concrete itself on a domestic slab.
- Why does the supplier want the order in 0.1 cubic metre steps?
- Batching plants weigh out cement, sand, aggregate and water to a recipe and dispense in fixed increments, so orders are placed in tenths of a cubic metre. That is why this calculator rounds the order up rather than quoting three decimal places you cannot actually buy.
- Can I pour a slab in two loads on different days?
- Not without planning for it. Concrete placed against concrete that has already set forms a cold joint, which is a plane of weakness and a likely crack. If a slab must be poured in stages, the joint has to be designed as a construction joint with dowels or a keyed edge, which is an engineering decision rather than a site improvisation.
Use this with
Concrete Slab Calculator (cubic metres and bags) answers one part of the question. These pick up where it stops: Concrete Volume Calculator, Brick and Block Calculator, Deck Materials Calculator, Mulch, Soil and Gravel Calculator and Tile Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Australian Building Codes Board: National Construction Code. Referenced standards for residential slabs and footings
- Standards Australia. AS 2870 residential slabs and footings, AS 3600 concrete structures
- Cement Concrete and Aggregates Australia. Guidance on concrete placement, mix selection and density
Concrete Slab Calculator (cubic metres and bags) produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.