How the concrete slab calculation works
Concrete is ordered by volume, so the job is to turn your slab dimensions into cubic metres. Multiply the length by the width to get the area, then multiply by the thickness in metres. A 6 by 4 metre slab at 100 mm is 6 × 4 × 0.1, which is 2.4 cubic metres.
volume (m³) = length (m) × width (m) × thickness (m)
If the slab has a thickened edge, which engineers call an edge beam or footing, that concrete sits below the slab and has to be added. The calculator takes the perimeter, multiplies by the beam width and the depth below the slab, and adds it in. It ignores the small double-count at each corner, which is a few litres and is well inside the waste allowance.
Why builders add waste
The calculated volume assumes perfect formwork on perfectly level ground. Neither exists. The ground under a slab is scraped, not machined, so the real thickness wanders between 95 and 115 mm across a slab that was meant to be 100. Formwork bows outward under the weight of wet concrete. Some concrete stays in the chute and on the tools. Allowing 5 to 10 percent covers all of that. Running short in the middle of a pour is expensive, because the plant charges a short-load fee for the top-up and the second truck may arrive after the first pour has started to set.
Bags or premix?
Bagged concrete is convenient for a few post holes or a small pad. It stops making sense quickly. A 20 kg bag yields about 9 litres, so a single cubic metre is around 108 bags, or roughly two tonnes of bags to carry, mix and place before the first batch goes off. Once you are over about half a cubic metre, a premix delivery is cheaper per cubic metre, the mix is consistent, and it arrives ready to place.
| Slab | Volume | 20 kg bags | Sensible option |
|---|---|---|---|
| 1 × 1 m pad, 100 mm | 0.10 m³ | 11 | Bags |
| 3 × 3 m shed, 100 mm | 0.90 m³ | 97 | Premix, or a borderline bag job |
| 6 × 4 m shed, 100 mm | 2.40 m³ | 259 | Premix |
| 6 × 6 m garage, 125 mm | 4.50 m³ | 484 | Premix, one truck |
Volumes in the table are before waste.
Ordering from the plant
Concrete plants supply in 0.2 cubic metre steps and most charge a short-load fee below about 3 cubic metres. An agitator truck typically carries 6 to 8 cubic metres, so a normal house slab is one or two trucks. When you order, they will ask for the strength grade, usually N20 or N25 for residential work, the slump, and whether you need a pump. Tell them the pour time, and have the formwork, mesh and bar chairs finished before the truck is due. Waiting time is charged.
Slab thickness and mesh
A slab for a shed, path or patio on good ground is normally 100 mm thick with SL72 mesh sitting on bar chairs so it ends up in the top third of the slab. Anything carrying a car is normally 125 mm or thicker, and the mesh may step up to SL82. A slab under a house is different again: it is designed by an engineer to AS 2870 for the site's soil classification, with edge beams and internal beams sized to that design. This calculator will give you the volume for any of those, but the thickness, beam sizes and mesh should come from the drawing, not from a rule of thumb.
Worked example
A 6 × 4 m shed slab, 100 mm thick, with a 300 × 300 mm edge beam below the slab all the way round. The slab itself is 6 × 4 × 0.1 = 2.40 m³. The perimeter is 20 m, so the beams are 20 × 0.3 × 0.3 = 1.80 m³. Together that is 4.20 m³. With 10 percent waste it comes to 4.62 m³, so the order is 4.8 m³, which fits in one truck. The 24 m² of slab needs two sheets of SL72.
Common questions
How many 20 kg bags of concrete make a cubic metre?
About 108. A 20 kg bag yields roughly 9.3 litres once mixed. Brands differ a little, so check the yield printed on the bag.
How thick should a concrete slab be?
100 mm for a shed, patio or path on good ground, with SL72 mesh. 125 mm or more if a car drives on it. House slabs are designed by an engineer to AS 2870 for the soil class.
How much waste should I allow?
5 to 10 percent. Formwork is never perfectly level and the ground is never perfectly flat, so the pour is always more than the drawing.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes. An agitator truck usually carries 6 to 8 cubic metres.
This calculator gives quantities from the dimensions you enter. It is not a structural design and it does not replace your engineer's drawing or your supplier's advice. Check the yield on your bags and confirm the order with the plant.