Concrete Calculator
Calculate how much concrete you need in cubic metres. Supports slabs, footings, columns, and stairs with 10% wastage allowance and pre-mix bag estimates.
Slab / Pad Dimensions
How to Use This Concrete Calculator
- Select the shape type that matches your project: Slab/Pad, Footing/Strip, Column/Pier, or Stairs.
- Enter the dimensions in the fields provided. Lengths are in metres, while thickness, width (for footings), depth, and diameter are in millimetres.
- The concrete volume is calculated instantly in cubic metres (m³) and displayed in the results panel.
- Use the "With 10% Wastage" figure as your recommended order quantity to account for spillage and variations.
- For small jobs, check the pre-mix bag count. For larger pours (over 0.5 m³), consider ordering ready-mix concrete delivery.
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Frequently Asked Questions
How much concrete do I need for a slab?
To calculate concrete for a slab, multiply the length (m) by the width (m) by the thickness (converted to metres). For example, a 4m x 5m slab at 100mm thick requires 4 x 5 x 0.1 = 2.0 cubic metres. Always add 10% for wastage, bringing the order to 2.2 m³. This formula works for any rectangular slab including paths, patios, driveways, and shed floors.
What is the standard wastage allowance for concrete?
The Australian industry standard wastage allowance for concrete is 10%. This accounts for spillage during pouring, uneven subgrade, over-excavation, and minor variations in formwork dimensions. For complex pours, difficult site access, or irregular shapes, some contractors allow up to 15%. Running short on concrete mid-pour is far more costly than ordering a small surplus.
How many bags of concrete per cubic metre?
You need approximately 111 x 20kg bags of pre-mix concrete per cubic metre. Each 20kg bag yields roughly 0.009 m³ of mixed concrete. Pre-mix bags (available from Bunnings, Mitre 10, and hardware stores) are suitable for small jobs like post holes and minor repairs. For anything over 0.5 m³, ready-mix delivery from a concrete plant is generally more economical and produces a more consistent result.
What thickness should a concrete slab be?
In Australia, residential concrete slabs are typically 100mm thick for paths, patios, and shed floors. Driveways and garage floors should be at least 100-150mm thick with steel reinforcement (SL72 or SL82 mesh). Structural slabs for house foundations are typically 100-150mm on waffle pods or 200-300mm for strip footings, as specified by a structural engineer to comply with AS 2870 (Residential slabs and footings).
Ordering Concrete in Australia
In Australia, concrete is measured and ordered in cubic metres (m³). When ordering ready-mix concrete from a batching plant, you will typically specify the volume in cubic metres, the concrete grade (e.g., N20 for general residential use, N25 for driveways, or N32 for structural applications), the slump (workability, usually 80-100mm for residential slabs), and any additives such as accelerators for cold weather pours or retarders for hot conditions. Most plants have a minimum order of around 0.2 to 0.5 m³, with a short-load fee applied for orders under 3 m³ or so.
For small projects such as fence post holes, letterbox footings, or small repair patches, pre-mix bags from hardware stores are a practical option. These come in 20kg bags (roughly 0.009 m³ each) and are mixed on-site with water. The trade-off is cost and effort: mixing 50 or more bags by hand is labour-intensive and more expensive per cubic metre than ordering ready-mix delivery. As a rough guide, if your project requires more than 0.5 m³, ready-mix is almost always the better choice.
Typical Australian residential applications include 100mm concrete paths and patios on compacted crushed rock base, 100-150mm driveways with SL72 or SL82 reinforcing mesh, strip footings (typically 300-450mm wide and 300mm deep for single-storey homes), and pier/column footings for decks and pergolas. All structural concrete work should be designed by a qualified engineer and must comply with the National Construction Code (NCC), AS 2870 for residential footings, and AS 3600 for concrete structures. Council approval may be required depending on the scope of works and your local authority requirements.