How to Calculate Concrete for Australian Projects
Getting your concrete quantities right before a project saves money and prevents costly shortfalls mid-pour. Whether you're laying a shed slab, footings for a deck, fence post holes or an exposed aggregate driveway, the core calculation is always volume: length × width × depth, measured in metres, giving cubic metres (m³).
Australian ready-mix concrete is priced per cubic metre, typically ranging from $180 to $300/m³ depending on your location, grade, and whether you need a pump truck. Pre-mix bags (20kg or 25kg from hardware stores like Bunnings) are more convenient for small jobs but significantly more expensive per cubic metre — often 3–5× the cost of ready-mix for the same volume.
Standard Concrete Slab Thicknesses in Australia
AS 2870 (the Australian standard for residential footings and slabs) gives guidance on slab depths, but common thicknesses used in practice are:
- Footpaths and garden paths: 75–100mm
- Shed or garage slabs: 100mm (minimum), 125mm for heavier use
- Driveways: 100–125mm for standard vehicles; 150mm if heavy vehicles expected
- House slabs: 85–100mm for the slab itself (plus edge beams 300–600mm deep)
- Post footings: Typically 300–400mm diameter × 600mm+ deep
Worked Example 1: Backyard Shed Slab
You're pouring a slab for a 6m × 3m garden shed at 100mm (0.1m) thick.
- Volume: 6 × 3 × 0.1 = 1.8m³
- With 10% waste: 1.98m³ → order 2.0m³
- Ready-mix at $220/m³: approximately $440
- In 20kg bags: ~170 bags at roughly $12 each = $2,040 (much more expensive)
For anything over about 0.5m³, ready-mix is almost always the more economical choice — even factoring in minimum delivery charges (often $200–400 for small loads under 3m³).
Worked Example 2: Deck Footings
A deck requires 6 circular post footings, each 300mm diameter and 600mm deep.
- Volume per footing: π × (0.15)² × 0.6 = 0.042m³
- Total for 6 footings: 0.252m³
- With 10% waste: 0.277m³
- In 20kg bags (~0.011m³ each): approximately 26 bags
For a job this small, pre-mix bags make sense. You can mix them one by one in a wheelbarrow or small mixer, which gives you better control over pour timing.
Ordering Ready-Mix: What to Know
When ordering ready-mix concrete in Australia, you'll need to specify:
- Grade (MPa): N20 is standard for residential slabs; N25 for driveways; N32 for structural elements. Higher MPa = stronger and more expensive.
- Slump: How workable the mix is. 80mm slump is standard; 100–120mm for tighter pours. Higher slump = wetter, easier to pour but slightly weaker.
- Volume in m³: Most suppliers have a minimum order of 0.2–0.3m³; small loads (under 3m³) often attract a surcharge.
- Delivery access: Concrete trucks need reasonable access — they're heavy. A pump may be needed for difficult access, adding $500–$1,500 to the job.
Does Concrete Shrink When It Sets?
Yes — concrete shrinks slightly as it cures, typically by 0.04–0.08%. For a 6m slab, that's about 2–5mm of total shrinkage. This is why control joints are cut into large slabs — to give the concrete a predetermined place to crack rather than cracking randomly. For calculation purposes, shrinkage is too small to affect your volume estimate, but it's worth knowing so you don't over-pour trying to compensate.
Mixing Concrete by Hand vs. Machine
For small jobs using pre-mix bags, you can mix by hand in a wheelbarrow — though it's hard work. A hired electric mixer (around $50–80/day from Kennards or similar) makes the job far easier and produces a more consistent mix. Always add water gradually; too much water is the most common cause of weak concrete in DIY projects. The mix should be workable but not sloppy — it should hold its shape when scooped with a shovel.
5 FAQs About Concrete in Australia
Standard concrete weighs approximately 2,400kg per cubic metre (2.4 tonne/m³). This is important to know for structural loading calculations and for checking that your subgrade can support the slab. Lightweight concrete mixes exist but are not commonly used in residential work.
Concrete reaches approximately 70% of its design strength after 7 days and 99%+ after 28 days. In hot, dry conditions (common in inland Australia), it cures faster but can crack if not kept moist — wet the surface regularly or cover with plastic sheeting for the first week. In cold conditions, curing slows significantly below 10°C.
Depends on the project and your council. Garden sheds under 10m² typically don't need a permit in most states. Driveways, carports and structural slabs usually do. In Victoria, "exempt development" rules allow some slabs without a permit; in NSW, SEPP (Exempt and Complying Development) codes apply. Always check with your council before pouring.
One 20kg bag of pre-mix concrete yields approximately 0.009–0.011m³ (9–11 litres) of mixed concrete. So 1m³ requires roughly 90–110 bags. Use the upper figure to be safe. At Bunnings prices (~$7–12 per bag), that makes bagged concrete 3–5× more expensive per cubic metre than ready-mix.
Cement is an ingredient in concrete — specifically the binding agent (typically Portland cement). Concrete is the finished mix of cement, water, sand (fine aggregate) and gravel or crushed rock (coarse aggregate). When people say "cement slab" they usually mean a concrete slab. Pre-mix bags contain dry cement, sand and aggregate already blended — you just add water.