How to Calculate Concrete for a Slab in Australia
Calculating the right amount of concrete for your slab project is straightforward once you understand the basic formula. Whether you're pouring a shed base, patio, or driveway, the process is the same: multiply length by width by depth to get your volume in cubic metres.
The Volume Formula
The formula for calculating concrete volume is Length × Width × Depth. All measurements should be in metres for the result to be in cubic metres (m³). For example, a slab measuring 3 metres long, 3 metres wide, and 100mm (0.1 metres) deep would require 3 × 3 × 0.1 = 0.9 cubic metres of concrete.
Standard Slab Thickness in Australia
The thickness of your slab depends on its intended use. For garden paths and light foot traffic, 75 to 100mm is generally sufficient. Patios and shed bases typically require 100mm thickness, which has become the standard for most residential applications in Australia. Driveways need to be thicker at 100 to 150mm to handle vehicle loads, with 125mm being a common recommendation. Garage floors usually require 100 to 125mm, though some councils may specify different requirements.
Why Add 10% Wastage?
Adding 10% wastage to your concrete calculation is standard practice for good reason. Ground is rarely perfectly level, meaning some areas will need more concrete to fill low spots. Some concrete is inevitably lost through spillage during pouring and spreading. A small amount sticks to mixing equipment and wheelbarrows. Minor measurement variations can also affect the final volume needed. For uneven sites or complex shapes, consider increasing wastage to 15%.
Converting to Bags
Boral, one of Australia's leading concrete suppliers, recommends approximately 108 bags of 20kg concrete mix per cubic metre. This calculator uses this figure to estimate the number of bags you'll need. Keep in mind that different brands may have slightly different coverage rates, so always check the manufacturer's recommendations on the bag.
Bags vs Ready-Mix
For small projects under 0.3 cubic metres, bagged concrete from Bunnings or your local hardware store is usually the most practical option. You can work at your own pace and don't need to worry about truck access. For larger projects over 0.5 cubic metres, ready-mix concrete delivered by truck becomes more cost-effective and saves considerable time and physical effort. Our bags vs ready-mix comparison calculator can help you determine the best option for your specific project.
Common Slab Projects
Australian homeowners commonly pour concrete slabs for shed bases, which typically range from 3×3 metres to 6×4 metres depending on the shed size. Carport slabs need to be sized for your vehicle with some extra space on each side. Patio extensions are popular for outdoor entertaining areas and usually connect to existing house slabs. Garden paths generally run 1 to 1.2 metres wide and can vary significantly in length depending on your property layout.