Concrete Slab Calculator
Order once, pour once — running short means a cold joint.
Work out the concrete volume in cubic metres, the number of 20 kg bags and the ready-mix cost for any slab, driveway or path. Enter your slab size and depth and the calculator adds a waste allowance, then shows the bags versus a ready-mix truck.
Estimates from slab geometry and typical AU prices — confirm the mix, reinforcement and delivered price with your engineer and batching plant.
How the concrete volume is worked out
Volume = length × width × depth, then add waste
The calculator takes your slab's plan area (length × width) and multiplies it by the depth in metres to get the raw volume, then adds any extra volume for footings or post holes. It then adds a waste allowance — the concrete you actually order is volume × (1 + waste). A 20 kg bag of pre-mixed concrete yields only about 0.01 m³, so a cubic metre is roughly 100 bags — nearer 110 once you include waste.
Worked example
A 20 m² floor poured 100 mm deep is 20 × 0.1 = 2.0 m³ before waste. Add the standard 10% and you order about 2.2 m³ — that's roughly 220 × 20 kg bags, or about $770 as ready-mix at $350/m³. Depth drives the number directly: the same slab at 150 mm would be 50% more concrete.
How deep should the slab be?
Depth is the single biggest lever on volume. Common Australian residential thicknesses are:
- 75 mm — garden paths and footpaths.
- 100 mm — house slabs, driveways and shed floors on stable ground.
- 125–150 mm — reactive clay sites, or slabs carrying heavy loads.
These are typical figures, not a specification. A structural slab — anything the house or a heavy load sits on — must be designed to AS 2870 for the actual soil class and built under the NCC. Get the depth, reinforcement and footing details from your engineer; don't size a structural pour off a rule of thumb.
Ready-mix vs bags, minimum loads and reinforcement
Bags vs a concrete truck
Bagged concrete works out to roughly $850–$1,300 per cubic metre once you count the bags, and it's a lot of hand-mixing. Ready-mix is about $320–$490 per cubic metre delivered (an N25 mix is often around $340–$370). Bags only make sense for small jobs — post holes, a small landing — where a truck's fees don't stack up. Past about a cubic metre, order ready-mix.
Minimum loads & short-load fees
Concrete trucks carry up to about 8 m³ but charge a minimum load (often around 1 m³) and a short-load surcharge under roughly 2–3 m³, plus delivery and any waiting time — so a small pour costs more per cubic metre than a big one. Ask for the price delivered, and batch pours where you can.
What this tool does not size
This calculator estimates volume and a rough cost only. You still need to specify the mix strength (MPa) and slump, and any reinforcing mesh — for example SL72 — with your engineer and batching plant. Reinforcement and mix spec don't change the volume, but they're essential to a slab that lasts.
❓ Frequently asked Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × depth in metres to get the volume, then add 5–10% for waste. A 4 × 5 m slab at 100 mm is 20 m² × 0.1 m = 2.0 m³ before waste, or about 2.2 m³ once you add the standard 10% for spillage and an uneven subgrade. Round the order up rather than down.
How many 20 kg bags per cubic metre?
A 20 kg bag of concrete mix yields roughly 0.01 m³, so you need about 100 bags per cubic metre — around 110 once you allow for waste. Bags suit small pours; past about a cubic metre, ready-mix is cheaper and far less mixing.
How deep should a concrete slab be?
Common Australian residential thicknesses are 75 mm for garden paths and footpaths, 100 mm for house slabs, driveways and shed floors, and 125–150 mm on reactive clay or under heavy loads. A structural slab's depth must follow your engineer or AS 2870 — it is not a rule-of-thumb guess.
Should I use bags or ready-mix?
Bagged concrete costs roughly $850–$1,300 per cubic metre and is a lot of hand mixing; ready-mix is about $320–$490 per cubic metre delivered. Bags only win for small jobs — post holes, a small landing — where a truck's minimum-load and short-load fees don't stack up. Over about a cubic metre, order ready-mix.
How much extra for waste?
Add 5–10% to the theoretical volume for spillage and an uneven or over-dug subgrade, and round the order up. Running short mid-pour means a cold joint or a second delivery fee, so a little concrete left over is cheaper than stopping the pour.
Does it include reinforcing mesh or the mix strength?
No. This tool sizes the concrete volume and gives a rough cost. You still need to specify the mix strength (for example N20 for paths, N25 for house slabs, N32 for heavy-duty work), the slump, and reinforcement such as SL72 mesh with your engineer or batching plant — none of that changes the volume.
Where these figures come from
This is a geometry-based estimate — the concrete volume comes from your slab's dimensions, not from any external rate table. The guidance below reflects Australian standards and typical trade practice.
- Residential slab design — slabs and footings for houses in Australia are designed to AS 2870 (Residential slabs and footings), which sets the classification of reactive soils and the slab type. Construction is regulated under the National Construction Code (NCC).
- Bag yield and quantities — a 20 kg bag of concrete mix yields about 0.01 m³, so roughly 100 bags per cubic metre; confirm the yield printed on the bag you buy.
- Prices — ready-mix around $320–$490/m³ delivered and bagged concrete around $850–$1,300/m³ are approximate market ranges that vary with region, mix strength, load size and delivery; they are not a quote.
Last checked: July 2026. Prices are indicative and vary by region, mix and site. Always confirm the mix strength, reinforcement and a delivered price with your engineer and batching plant.
Select the question that matches where you are right now.
The headline number is the concrete to order: the slab's volume plus a waste allowance for spillage and an uneven subgrade. The breakdown also converts it to 20 kg bags and, if you enter prices, to a ready-mix and a bagged cost.
Order this volume from the batching plant (or buy the bags). Give them the mix strength and slump your engineer specifies — the volume is what you order, the mix is a separate line.
It's not a structural design or a full quote. It doesn't size reinforcement, footings depth, or the mix strength, and it doesn't include pump hire, formwork or labour.
The volume maths (area × depth) is exact; real pours vary with an uneven subgrade and spillage, which is what the waste allowance absorbs. Confirm the bag yield and delivered price with your supplier.
Four things move the volume: the area, the depth, the waste allowance, and any extra volume for footings.
Volume scales straight with depth: a slab at 150 mm is 50% more concrete than the same slab at 100 mm. Getting the depth right — from AS 2870 for a structural slab — matters more than any other input.
5–10% covers spillage and an over-dug or uneven subgrade. Lean towards 10% on rough ground — a subgrade 20 mm deeper than planned across a big slab is real extra concrete.
Slabs are rarely a flat rectangle. Edge beams, thickenings under walls and separate post holes all add concrete — a 300 mm-diameter post hole 600 mm deep is about 0.042 m³. Add these under the extra-volume field so the order isn't short.
A few habits stop you over- or under-ordering.
Measure the actual formed-up area and check the subgrade depth at several points — over-dig adds concrete. Use the real dimensions and let the waste allowance cover the rest.
Running short mid-pour means a cold joint or a second short-load delivery fee. A little concrete left over is always cheaper than stopping the pour.
Volume is only half the order. Tell the plant the MPa (N20–N32), slump and any reinforcement (e.g. SL72 mesh) your engineer specifies before the truck rolls.
A slab is usually one line in a bigger renovation budget. Model the surface and the money side too.
Check your borrowing capacity before you commit to a bigger job.
Borrowing capacity →See what monthly repayments a renovation adds to your home loan.
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