Use this concrete slab calculator to work out exactly how much concrete a slab needs — in cubic metres, reinforcing mesh sheets, premix bags and delivered cost. It handles rectangular, L-shaped, round and thickened edge beam slabs.
Slab 1
Advanced options
Trades allow 5–10%. Use 15% on an unformed base.
Sheets are 6.0 × 2.4 m. Laps are allowed for.
Yields vary by brand — check the bag.
Supply only. Excludes pump and short-load fees.
The short answer
A concrete slab calculation is length × width × thickness, with every measurement converted to metres. A 6 m × 4 m slab at 100 mm thick is 6 × 4 × 0.1 = 2.4 m³. Add 10% for waste and order 2.7 m³.
That same concrete slab needs roughly 2 sheets of SL72 mesh and, at $320/m³, costs about $860 in supply alone.
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How the concrete slab calculator works
A concrete slab is a rectangular prism, so calculating concrete for a slab is one multiplication: area times thickness. What trips people up is that slab length and width are measured in metres while slab thickness is quoted in millimetres. Multiply 6 × 4 × 100 and you get 2,400 — a meaningless number. Convert the thickness to metres first and the concrete slab calculator returns the 2.4 m³ you actually need.
Slab volume (m³) = Length (m) × Width (m) × Thickness (m)
Thickness in metres = millimetres ÷ 1000
85 mm = 0.085 m · 100 mm = 0.1 m · 125 mm = 0.125 m · 150 mm = 0.15 m
A useful shortcut once you know your slab thickness: at 100 mm, every square metre of slab takes exactly 0.1 m³ of concrete. So a 24 m² slab needs 2.4 m³. At 150 mm it’s 0.15 m³ per square metre, and so on. That single relationship lets you sanity-check any concrete slab calculator result in your head before you order.
Worked example — 6 × 4 m garage slab at 100 mm
Batch plants sell in 0.1 m³ increments, so always round the concrete slab volume up to the next tenth.
Calculating L-shaped and round concrete slabs
Not every slab is a rectangle. The concrete slab calculator above handles both of these shapes directly, but here is what it does behind the scenes.
L-shaped slabs
Split the L into two rectangles, calculate each area separately, add them, then multiply the combined area by slab thickness. The trap is double-counting the overlap where the two rectangles meet — always draw the split so the sections butt against each other rather than overlap.
Volume = (A₁ × B₁ + A₂ × B₂) × Thickness
6 × 4 m section = 24 m²
3 × 2.5 m section = 7.5 m²
Total 31.5 m² × 0.1 m = 3.15 m³
Round slabs
Circular slabs — tank bases, round patios, silo pads — use the circle area formula. The usual error is entering the diameter where the formula wants the radius, which returns four times too much concrete.
Volume = π × (Diameter ÷ 2)² × Thickness
4 m diameter at 100 mm
= 3.1416 × 2² × 0.1
= 1.26 m³
Concrete slabs with thickened edge beams
This is where most online concrete slab calculators fall short, and where people most often under-order. A house slab or shed slab on ground is rarely a flat plate. It has a thickened edge beam running around the perimeter — and often internal beams too — that carries the load down to founding material.
Those beams can hold as much concrete as the slab itself. A 10 × 8 m slab at 100 mm is 8 m³ as a flat plate. Add a 300 mm wide × 300 mm deep edge beam around the perimeter and you add roughly another 3 m³, which is a 38% increase. Order the flat-plate figure and you will run short with a truck sitting in your driveway.
Volume = (Area × Slab thickness) + (Perimeter × Beam width × Beam depth)
10 × 8 m slab, 100 mm thick, 300 × 300 mm edge beam
Plate: 80 × 0.1 = 8.0 m³
Beam: 36 m perimeter × 0.3 × 0.3 = 3.24 m³
Less corner overlap ≈ 0.11 m³
= 11.13 m³ total
Edge and internal beam dimensions for a house slab are set by the site’s soil classification under AS 2870. On a Class H1, H2 or E reactive clay site those beams can be 600 mm or deeper, and the difference between a Class A and a Class E design on the same footprint can be several cubic metres. Use this concrete slab calculator to price and order once you have engineered dimensions — not to decide what those dimensions should be.
How thick should a concrete slab be?
Slab thickness drives volume more than any other input, so settle it before you calculate. These are common Australian residential figures.
| Slab application | Thickness | Concrete per m² | Typical mesh |
|---|---|---|---|
| Garden path, mower strip | 75–85 mm | 0.075–0.085 m³ | SL62 |
| Footpath, pedestrian only | 85–100 mm | 0.085–0.1 m³ | SL62 |
| Shed floor | 100 mm | 0.1 m³ | SL72 |
| Garage slab | 100 mm | 0.1 m³ | SL72 |
| Residential driveway | 100–125 mm | 0.1–0.125 m³ | SL72–SL82 |
| Caravan or truck access | 150 mm | 0.15 m³ | SL82–SL92 |
| House slab on ground | 100 mm + beams | Engineer designed | Per design |
Going thicker is not always better. A thicker concrete slab on a poorly compacted base still cracks — the base does more work than the extra 25 mm ever will. Spend the money on preparation and reinforcement before you spend it on depth.
How much reinforcing mesh does a concrete slab need?
Standard Australian reinforcing mesh sheets are 6.0 m × 2.4 m, giving 14.4 m² gross. Sheets must lap at joints — typically one full square plus 25 mm, around 200–300 mm — so effective coverage is closer to 13 m² per sheet. The concrete slab calculator above uses that figure, which is why its sheet count is higher than simply dividing area by 14.4.
Sheets = Slab area ÷ 13.0 m², rounded up
24 m² garage slab ÷ 13.0 = 1.85 → 2 sheets
80 m² house slab ÷ 13.0 = 6.15 → 7 sheets
| Mesh grade | Bar / spacing | Typical slab use |
|---|---|---|
| SL62 | 6.0 mm @ 200 mm | Paths, light-duty slabs, mower strips |
| SL72 | 7.6 mm @ 200 mm | Standard residential slab, shed and garage floors |
| SL82 | 7.6 mm @ 200 mm heavier | Driveways, slabs taking vehicle loads |
| SL92 | 8.6 mm @ 200 mm | Heavy-duty and commercial slabs |
Mesh only works where it sits. It belongs in the upper third of the slab with a minimum of 20–30 mm cover, held there on bar chairs at roughly 800 mm centres. Mesh lying on the plastic at the bottom of a concrete slab does almost nothing — this is the single most common reinforcement failure on DIY slabs.
Concrete needed for common slab sizes
Quick reference at 100 mm thickness, before waste. Run your own dimensions through the concrete slab calculator above for an exact figure including mesh and cost.
| Slab size | Area | Concrete | Order with 10% | SL72 sheets |
|---|---|---|---|---|
| 3 × 3 m garden shed | 9 m² | 0.9 m³ | 1.0 m³ | 1 |
| 4 × 3 m small shed | 12 m² | 1.2 m³ | 1.4 m³ | 1 |
| 6 × 3 m single garage | 18 m² | 1.8 m³ | 2.0 m³ | 2 |
| 6 × 4 m large shed | 24 m² | 2.4 m³ | 2.7 m³ | 2 |
| 6 × 6 m double garage | 36 m² | 3.6 m³ | 4.0 m³ | 3 |
| 9 × 6 m workshop | 54 m² | 5.4 m³ | 6.0 m³ | 5 |
| 12 × 8 m large shed | 96 m² | 9.6 m³ | 10.6 m³ | 8 |
Types of concrete slab used in Australia
Most common
Stiffened raft
Flat plate with thickened edge and internal beams poured monolithically. The default residential slab across most of Australia.
Reactive sites
Waffle pod
Polystyrene voids form a grid of ribs above ground level. Uses less concrete than a raft of equal stiffness and needs no excavation for beams.
Sheds & garages
Slab on ground
Simple flat plate, usually 100 mm with a modest thickened edge. What most people mean by a concrete slab.
The distinction matters for your calculation. A waffle pod slab’s volume cannot be found with a simple area × thickness formula, because the pods displace a large proportion of the concrete — you need the rib layout and pod dimensions from the engineering drawing. A stiffened raft is calculable using the edge beam mode in the concrete slab calculator above, provided you have the beam dimensions.
What goes under a concrete slab
The slab is the last step. Ordering concrete before the base is right is how good concrete ends up in a cracked slab.
- Strip and level. Remove topsoil and organic material entirely — a slab sitting on grass or fill will settle unevenly.
- Compacted fill. 50–100 mm of crushed rock or road base, compacted in layers with a plate compactor. This layer does more for crack resistance than slab thickness.
- Vapour barrier. 0.2 mm polythene, taped at joints, lapped 200 mm and turned up at the edges. Allow around 15% extra area for laps.
- Formwork. Braced against 2.4 tonnes per cubic metre of hydraulic pressure. Under-braced forms blowing out mid-pour is the most common DIY slab failure.
- Mesh on chairs. Upper third of the slab, 20–30 mm cover, chairs at roughly 800 mm centres.
- Pour, screed, float, cure. Keep it damp for at least seven days — a slab that dries too fast is permanently weaker.
What does a concrete slab cost?
The concrete slab calculator above returns supply-only cost, because that is the number you control when ordering. Supply is roughly a third of a finished slab price once you add preparation, formwork, mesh, labour and finishing.
| Cost component | Typical AU range |
|---|---|
| Ready-mix concrete supply | $250–$380 / m³ |
| Reinforcing mesh sheet (SL72) | $90–$140 / sheet |
| Concrete pump hire | $700–$1,400 / day |
| Short-load fee under ~2 m³ | $150–$300 |
| Plain slab supplied and laid | $85–$150 / m² |
Prices vary significantly by state and by distance from the batch plant. Always get a quote for your actual postcode before you budget — and note that Saturday pours usually carry a surcharge.
Concrete slab calculation mistakes to avoid
- Treating a house slab as a flat plate. Edge and internal beams are frequently 30–50% of the total concrete volume.
- Mixing units. Metres multiplied by millimetres gives a figure 1,000× too large.
- Dividing slab area by 14.4 for mesh. Laps mean effective coverage is closer to 13 m² per sheet.
- Forgetting the waste allowance and running short the moment the sub-base has a low spot.
- Using diameter instead of radius on a round slab — four times too much concrete.
- Ignoring the pump line. Around 0.25 m³ stays in the hose and never reaches the slab.
- Rounding down. Extra concrete costs a few dollars; a cold joint through the middle of a slab is permanent.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply slab length by width to get the area in square metres, then multiply by the slab thickness converted to metres. A 6 m × 4 m slab at 100 mm is 6 × 4 × 0.1 = 2.4 m³. Add 5–10% for waste and round up to the next 0.1 m³ when ordering.
How much concrete do I need for a 6 × 4 m slab?
At 100 mm thick, a 6 m × 4 m concrete slab needs 2.4 m³ before waste, or 2.7 m³ with a 10% allowance. It also needs about 2 sheets of SL72 mesh. At 125 mm thickness the same slab takes 3.0 m³ before waste.
How thick should a concrete slab be?
100 mm is standard for shed floors, garage slabs and most residential slabs. Paths can go down to 75–85 mm. Driveways run 100–125 mm, or 150 mm where trucks or caravans will drive on them. House slabs on ground are engineer-designed under AS 2870 and include thickened beams.
How many mesh sheets does a concrete slab need?
Divide the slab area by about 13 m² and round up. Sheets are 6.0 × 2.4 m — 14.4 m² gross — but must lap 200–300 mm at joints, so effective coverage is lower. A 24 m² slab needs 2 sheets; an 80 m² slab needs 7.
How much concrete is in a 10 × 8 m slab with edge beams?
The flat plate at 100 mm is 8 m³. A 300 mm wide by 300 mm deep perimeter beam adds roughly 3.1 m³ after allowing for corner overlap, giving about 11.1 m³ before waste. Beam dimensions must come from an engineer’s design, not an assumption.
How much does a concrete slab cost per square metre?
Supplied and laid, a plain concrete slab typically runs $85–$150 per square metre in Australia depending on access, thickness, finish and site preparation. Concrete supply alone is roughly $250–$380 per cubic metre, which is about $25–$38 per square metre at 100 mm thick.
What strength concrete should a slab be?
N25 suits most residential slabs, paths and shed floors. N32 is used for driveways and anything taking vehicle loads. N20 is generally limited to footings and non-structural work. You’ll also be asked for slump — 80–100 mm is typical for a slab.
Can I pour a concrete slab in sections?
A structural slab should be poured in one continuous operation. Stopping and restarting creates a cold joint — a permanent weak line where the first pour has stiffened before the second arrives. If a slab is too large for one pour, the joint must be a designed construction joint with dowels, not an accident of timing.
Does a concrete slab need to be thicker at the edges?
Yes for anything structural. A thickened edge beam carries load down to founding material and stiffens the slab against ground movement. Depth and width come from the site’s soil classification under AS 2870 — 300 mm is common on stable sites, but reactive clay can require 600 mm or more.
How long before you can build on a concrete slab?
A slab is walkable after 24–48 hours and can usually take light traffic at around 7 days, but it reaches specified strength at 28 days. Most builders start frame work at around 7 days. Keep the slab damp for the first week — curing matters more to final strength than most people expect.
Standards and sources
- AS 3600 — Concrete structures
- AS 2870 — Residential slabs and footings, including site classification
- AS/NZS 4671 — Steel reinforcing materials (mesh grades and sheet sizes)
- Australian ready-mix supplier product data sheets for bag yields and mix strengths
Related calculators
Concrete Slab Cost CalculatorSupplied and laid price per square metre
Concrete Bags CalculatorCubic metres to 20, 25 or 40 kg bags
Reo Mesh CalculatorSheets and laps for any slab area
Shed Slab CalculatorSizing and thickness for shed floors
Square Metre CalculatorWork out slab area first
How this concrete slab calculator works out its numbers
Slab volume uses standard geometric formulas with all inputs normalised to metres before multiplication. The edge beam mode adds perimeter × beam width × beam depth to the flat plate volume and subtracts the corner overlap so corners are not counted twice. Mesh sheet counts assume 6.0 × 2.4 m sheets at 90% effective coverage to allow for 200–300 mm laps. Wet density is taken as 2.4 t/m³ and truck capacity as a 6 m³ standard agitator. Premix bag yields are 0.010 m³ for 20 kg, 0.0125 m³ for 25 kg and 0.020 m³ for 40 kg — check your specific product, as some brands yield 0.009 m³.
Cost figures are supply-only indicative rates for Australian metropolitan areas and exclude pumping, short-load fees, out-of-hours surcharges, formwork, reinforcement and labour.
This concrete slab calculator produces quantities, not structural design. Slab thickness, reinforcement grade, beam dimensions and site classification for any structural slab must come from a qualified engineer working to AS 3600 and AS 2870. This is particularly important on reactive clay, sloping or filled sites.