How this calculator works
The formula is: length × width × thickness (in metres) = cubic metres. A 4 × 3 m shed base at 100 mm thick = 4 × 3 × 0.1 = 1.2 m³ of concrete. We add a 10% wastage margin on top — uneven ground, spillage and mixing variations all consume more than the theoretical volume.
For bags, we use the AU industry standard of 108 × 20 kg bags per cubic metre (based on Boral and Cement Australia product data). For ready-mix, the volume in m³ goes direct to your concrete supplier — they quote and deliver by the cubic metre.
Standard AU slab thicknesses
- 100 mm — garden paths, patio slabs, shed floors and workshop floors. The most common DIY slab thickness in Australia.
- 125 mm — residential driveways. Extra thickness handles vehicle loads without cracking.
- 150 mm — garage floors, heavier driveways. Standard for single and double garages where vehicles are parked regularly.
- 200 mm — footings for pergolas, decks, and structural posts. Also used for commercial slabs and slabs over poor or fill ground.
Bags vs ready-mix — which is right for your job?
The decision comes down to volume and convenience:
- Under 0.3 m³ (roughly 32 bags): bags from Bunnings. Mix in a wheelbarrow or hire a small electric mixer. Manageable for one person.
- 0.3 – 1.0 m³ (32–108 bags): bags are still feasible but hard work. A half-day job for two people. Consider a concrete mixer hire (~$80/day) to speed things up.
- Over 1.0 m³: order ready-mix. A concrete truck delivers pre-mixed concrete — you just pour and screed. Much less labour, consistent quality. Ready-mix in AU costs approximately $280–$350 per m³ delivered (2026). Note: most suppliers have a minimum order of 0.5–1.0 m³, and loads under 3 m³ attract a short-load fee of $50–100.
For a typical 6 × 4 m shed base at 100 mm, you're looking at 2.64 m³ — firmly in ready-mix territory. Budget approximately $800–$950 for the concrete alone, plus pump hire ($150–300) if the truck can't reach the pour location.
Reinforcement — do you need it?
Most residential slabs in Australia use SL62 or SL72 reinforcing mesh placed in the middle third of the slab (typically 50 mm up from the bottom). Mesh is particularly important for:
- Driveways and garage floors (vehicle loads)
- Slabs over fill or disturbed ground
- Slabs over 4 m in any direction (control cracking)
- Areas with reactive (clay) soils — common in SE Queensland
Garden paths and small patios on stable ground can often skip mesh, but it's cheap insurance (SL62 mesh is around $40–$60 per sheet at Bunnings) so most builders include it regardless.
Curing — don't rush it
Concrete reaches workable strength in 24–48 hours (foot traffic), usable strength in 7 days (drive on it), and full design strength at 28 days. Keep it damp for the first 7 days — wet it down once or twice a day or cover with plastic sheeting. In hot QLD conditions especially, concrete dries too fast and loses strength. Curing properly is as important as the pour itself.