Adelaide Earth Moving & Concrete
Adelaide Concrete Calculator: Estimate Your Project Needs
You buy concrete by the cubic metre, so running out mid-pour is the most expensive mistake a slab can suffer. Use the calculator below to estimate how much your slab, footing, path or driveway needs before you order. Then read on for how the maths works, how much extra to order, and when an estimate should become a site visit.
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How to Calculate Concrete Volume
Quick Answer
Basically, volume in cubic metres is length in metres, times width in metres, times depth in metres. For example, a 6 by 4 metre shed slab at 100 millimetres deep is 6 x 4 x 0.1 = 2.4 cubic metres. Then add ten per cent for wastage and uneven ground, so you order 2.6 to 2.8 cubic metres. Our concrete calculator below does the conversion for you, as well as handling round post holes and multiple sections.
The trap is units. People almost always talk about slab depth in millimetres, so 100 millimetres has to become 0.1 metres before you multiply. Otherwise the result is a thousand times too big. Another trap is assuming the ground is flat. A base that sits 10 millimetres low across a 60 square metre driveway quietly swallows 0.6 cubic metres you never ordered. That is exactly how pours run short.
Free Tool
Adelaide Concrete Calculator
Enter lengths in metres and depth in millimetres, then read the results, which update as you type.
The suggested order rounds up to the nearest 0.2 cubic metres, because that is how most Adelaide suppliers batch deliveries. Bag counts, meanwhile, assume roughly 0.01 cubic metres per 20 kilogram bag. For anything over about one cubic metre, however, ready-mix delivery is far more practical than bags.
Estimates only. Volumes assume a flat, compacted base as well as straight formwork. However, every real site has variation, which is why the wastage allowance exists. It is also why a fixed quote comes from a site visit rather than a calculator.
Calculating Slabs, Footings and Driveways
Slabs and paths
Length x width x depth. For an L-shaped or irregular slab, split it into rectangles, calculate each one and then add them together. Thickened edge beams are extra rectangles too. For example, a 300 millimetre wide beam dropping 200 millimetres below a 6 x 4 slab adds about 20 x 0.3 x 0.2 = 1.2 cubic metres.
Footings and piers
Strip footings are long, thin rectangles. For instance, a 400 x 400 millimetre footing running 30 metres is 30 x 0.4 x 0.4 = 4.8 cubic metres. Round piers use the circle formula instead: 3.14 x radius squared x depth, multiplied by the number of holes. Twelve 300 millimetre piers at 600 millimetres deep therefore come to about 0.51 cubic metres.
Driveways
Driveways are rarely one clean rectangle. Break the shape into rectangles and triangles, then include the crossover to the kerb. A triangle, in case you need it, is half the base times the height. Generally, use 100 millimetres for a standard car driveway, or 125 to 150 for heavier vehicles. A 60 square metre driveway at 100 millimetres is 6 cubic metres before wastage, which is roughly a full agitator load.
Typical depths for Adelaide projects
| Project | Typical thickness | Note |
|---|---|---|
| Garden paths and pool surrounds | 75 to 100 mm | 100 mm where wheelbarrows and mowers cross |
| Shed and garage slabs | 100 mm with thickened edges | Engineered to the soil classification on reactive clay |
| Residential driveways | 100 to 125 mm | 125 to 150 mm for caravans, boats and trade vehicles |
| Commercial hardstands | 150 mm and up | Designed around the heaviest regular vehicle |
| Footings and piers | To the engineer's design | Depth follows the soil, not a rule of thumb |
Measure twice, and measure the finished formwork rather than the sketch. For instance, a driveway drawn at 3 metres wide often forms up at 3.1 once the boxing goes in. On a 20 metre run, that extra 100 millimetres is another 0.2 cubic metres at 100 millimetres deep.
The Importance of Ordering Slightly More Concrete
Every experienced concreter orders more than the maths says, and it is not carelessness. To begin with, formwork bows outward under the weight of wet concrete. Compacted bases are never perfectly flat either. Some concrete also stays in the truck chute, the pump line and the barrows. Worse, if the load comes up short, the second delivery carries a minimum charge and a wait. Meanwhile, the first pour sits in the sun going off, so it forms a cold joint you will see for the life of the slab.
Slabs and driveways: add 10 per cent
On a well-prepared, compacted base with straight formwork, ten per cent covers the dips, the bow and the losses in handling.
Footings and trenches: add 10 to 15 per cent
Trench walls dug in clay are rarely straight, especially after rain. Consequently, a collapsed edge or an over-dig swallows concrete nobody measured.
Round up to the delivery increment
Suppliers batch in 0.2 cubic metre steps, so 2.64 becomes 2.8. For small jobs in bags, similarly, round up to the next whole bag and keep one spare.
A little left over costs a few dollars, and besides, it goes into a footing, a post hole or a run of garden edging. Running short, however, costs a second delivery fee, an hour of finishers standing around and a weak joint through the slab. So order the extra.
When the Calculator Is Not Enough
The concrete calculator gives you a volume, but it cannot see your ground. Adelaide's reactive clay soils, after all, shrink and swell with the seasons. For that reason, engineers design footings and house slabs to the site's soil classification rather than to a standard depth. Similarly, a shed slab on a poorly compacted base will crack no matter how accurately you ordered the concrete. Mesh size, joint spacing, mix strength and slump also change with the job.
For a driveway, a shed slab or anything structural, use the calculator to get a feel for the scale of the job first. Then have us stand on the site. Our shed and garage slabs Adelaide and concrete driveways Adelaide teams then quote volume, base, reinforcement and labour as one fixed number. Because the same crew handles earthworks and concrete, we plan the excavation and the pour together rather than guessing them separately.
FAQs
Adelaide Earth Moving & Concrete
Ready To Turn An Estimate Into A Quote?
Send us your dimensions, your address and what you are building, and we will take it from there. The Adelaide Earth Moving and Concrete team will then confirm the volume, check the ground and access, and quote the whole job from excavation to finished slab. Straight up pricing, no surprises.
