How Much Concrete Do You Need for a Slab? A Complete Australian Guide

Why Getting Your Concrete Volume Right Matters

Order too little and you'll face a costly second batch delivery — often with a visible cold joint where the two pours meet. Order too much and you're paying for concrete that hardens in the truck. In Australia, ready-mix concrete is priced per cubic metre and typically carries a minimum order fee, so even small miscalculations sting.

The good news: the maths is straightforward. Use our concrete calculator to get a precise figure, or follow the steps below to work it out yourself.

The Basic Formula

Volume (m³) = Length (m) × Width (m) × Thickness (m)

Say you're pouring a 6m × 4m slab at 100mm (0.1m) thick:

• 6 × 4 × 0.1 = 2.4 m³

Always add a 10% wastage allowance for spillage, uneven sub-base, and the fact that ground is never perfectly level. Round up to 2.64 m³ — order 2.7 m³ to be safe.

Recommended Slab Thickness by Application

Going below minimum thickness is a false economy — you'll end up with cracking and structural failure within a few years, especially in Queensland's reactive clay soils or Victoria's cold winters.

Concrete Mix Strength: What to Order

Australian concrete is rated by its 28-day compressive strength in megapascals (MPa). For most domestic slabs, the options are:

• 20 MPa — light foot-traffic paths and garden areas
• 25 MPa — standard residential slabs, driveways, sheds
• 32 MPa — heavy vehicle areas, commercial work, exposed aggregate
• 40 MPa — structural applications, high-load situations

If you're unsure, order 25 MPa for a standard domestic job. The price difference between 20 MPa and 25 MPa is usually around $5–$10 per m³ — a small investment for significantly better durability.

Reinforcement: Mesh vs Rebar

Unreinforced concrete slabs crack. Full stop. For any slab over 2m × 2m you should include reinforcement:

• SL72 mesh — standard for residential shed and driveway slabs. Place it at the mid-depth of the slab or slightly above centre.
• SL82 mesh — heavier option for larger spans or light commercial.
• Rebar (N12 or N16) — for structural slabs, footings, or any engineer-specified job.
• Fibremesh (polypropylene fibres) — added to the mix itself. Reduces surface cracking and plastic shrinkage cracks. A good addition for driveways exposed to temperature swings.

For most DIY shed slabs, a single layer of SL72 mesh with 40mm cover from the bottom is standard practice and will satisfy most building inspectors.

Sub-Base Preparation: The Step Everyone Skips

The most common cause of slab failure isn't poor concrete — it's poor ground preparation. Before you pour:

1. Excavate to the correct depth (slab thickness + at least 75mm for compacted roadbase)
2. Compact the sub-base using a plate compactor — don't rely on the original soil
3. Lay 75–100mm of compacted Class 2 roadbase (or crushed rock)
4. Install a 200-micron plastic vapour barrier (mandatory under any habitable slab, highly recommended everywhere else)
5. Set your formwork level and braced — check it twice before the truck arrives

Skipping the roadbase is the number one DIY mistake. No matter how good your concrete is, it will crack and sink if the ground underneath isn't solid and well-drained.

What Does a Concrete Slab Cost in Australia?

Ready-mix concrete in 2026 runs approximately $180–$250 per m³ depending on your state, mix strength, and distance from the batching plant. Expect a minimum order charge of around $200–$400 even if you only need half a cubic metre.

For a typical 6m × 4m shed slab (2.7 m³ at 25 MPa), budget roughly:

• Concrete: $500–$700
• Mesh reinforcement: $80–$120
• Roadbase: $60–$100
• Plastic and formwork timber: $40–$80
• Pump hire (if needed): $350–$600

For the right tools to get the job done, a quality concrete finishing set including bull float, hand float, and edger is worth having before the truck rolls in — you won't have time to figure out equipment once the pour starts.

Step-by-Step: Pour Day Checklist

1. Confirm your volume with the ready-mix plant the morning of the pour
2. Have at least one extra person on site — this is not a solo job
3. Wet the sub-base (but don't puddle it) before the truck arrives
4. Pour and spread quickly — concrete starts setting within 30–45 minutes in warm weather
5. Screed level using a long screed board across your formwork
6. Bull float to embed aggregate and smooth the surface
7. Cut control joints at 3m intervals once initial set begins (use an angle grinder or saw)
8. Cure for at least 7 days — keep moist or cover with plastic sheeting

Use our free concrete calculator to enter your exact dimensions and get the precise volume with wastage factored in. You can also cross-check your fencing post footings using the fencing calculator if you're combining both projects.