Concrete Slab Calculator
Concrete for a slab on ground, including the thickened edge beam that runs around the perimeter.
Your result
Enter your slab dimensions above to see how much concrete you need.
This works out how much concrete fills the shape you describe. It does not tell you how thick your slab should be, what reinforcement it needs, or whether the design suits your soil. Those depend on your site classification and what the slab carries, and they need your engineer or builder.
Why the edge is thicker than the slab
Most Australian slabs on ground are not a flat plate of concrete. The perimeter — and often internal lines under load-bearing walls — drops into a deeper beam, cast at the same time as the slab. That beam carries the wall loads down to founding material and stiffens the whole slab against the ground moving underneath it.
That extra concrete is easy to forget. A flat calculation on a 8 m × 6 m slab at 100 mm gives 4.8 m³. Add a 300 mm deep, 300 mm wide edge beam and you need about 5.7 m³ — nearly a cubic metre more, which is the difference between one delivery and an awkward second one.
How this calculator handles it
The slab body is length × width × thickness. The beam only adds concrete where it goes deeper than the slab already is, because the slab pour already fills the top of that trench. So the extra volume is the beam's footprint multiplied by the difference between the edge depth and the slab thickness.
The footprint is worked out as the whole slab area minus the untouched middle, which counts the corners once rather than twice. A naive "perimeter × beam width" over-orders, and the error grows with the beam width.
Enter the edge depth as the total depth from the top of the slab, not the extra bit below it. That is how slab details are normally dimensioned, and it is the number you will have on a drawing.
Common slab thicknesses
General examples of what gets used, not a specification for your job:
- 75–100 mm — paths, small garden slabs, light-duty paving.
- 100 mm — the common nominal thickness for a domestic slab on ground, shed floor or patio.
- 100–125 mm — garages and driveways taking cars.
- 150 mm and up — heavier vehicles, machinery, or poor ground.
The thickness that is right for your slab depends on the soil classification, the loads, the reinforcement and the relevant standards. Thicker is not automatically better either — a thick slab on unsuitable ground still cracks.
Things that change how much you actually need
- The ground is never flat. A slab cast on a surface that dips 20 mm over its length uses more concrete than the arithmetic suggests. This is the main reason to keep the waste allowance on.
- Formwork spreads. Timber forms bow slightly under the weight of wet concrete, and the slab ends up marginally bigger than the drawing.
- Internal beams. If your slab has thickened lines under internal walls, those are extra on top of the perimeter beam. Work them out with the footing calculator and add them.
- Sub-base compaction. Loose or uncompacted fill lets concrete sink into it.
Frequently asked questions
What if my slab has no thickened edge?
Leave both edge fields blank. You then get a plain length × width × thickness result, identical to the main concrete calculator.
Is edge depth measured from the top of the slab or the bottom?
From the top of the slab. A "300 mm edge beam" on a 100 mm slab drops 200 mm below the underside of the slab. Enter 300, not 200 — the calculator does that subtraction for you.
Does this work for a waffle raft slab?
No. Waffle rafts sit on top of the ground with void formers in a grid, and the concrete volume depends on the pod size, spacing and rib dimensions. This calculator assumes a conventional slab on ground with a perimeter beam. For a waffle raft, use the volume from your engineer's drawings.
Does reinforcement change the volume?
Not meaningfully. Steel mesh and bar displace a very small fraction of the concrete, far less than the waste allowance covers. Order on the concrete volume and ignore the steel.