Concrete Calculators Australia
Free metric concrete calculators built for Australian conditions. Work out how much concrete you need for a slab, footing or post hole — in cubic metres and bag counts. Calculate reinforcing bar quantities and weights. All calculators use Australian standard mixes, NCC-aligned guidance and metric inputs only.
Concrete volume calculators
Concrete Slab Calculator
Enter length, width and thickness in metric to get volume in m³ plus 20 kg and 25 kg bag counts.
Open calculator →Footing & Post Hole Calculator
Round post holes (diameter × depth) and rectangular pad footings. Per-footing and total volumes.
Open calculator →Concrete Bags Calculator
Enter a volume in m³ and get bag counts for 20 kg, 25 kg and 40 kg premix bags. Includes total weight.
Open calculator →Rebar Calculator
Calculate reinforcing bar quantity, lineal metres and weight (kg) for concrete slabs. AU bar sizes R10 to N32.
Open calculator →Driveway Concrete Calculator
Enter driveway length, width and thickness to get m³, bag counts and a ready-mix recommendation.
Open calculator →Australian concrete standards reference
- AS 3600 — Concrete structures. The primary Australian standard for structural concrete design, covering mix design, reinforcement cover, compressive strength grades (e.g. N20, N25, N32) and durability requirements.
- NCC (National Construction Code) — Volume One references AS 3600 for structural requirements. Volume Two sets minimum requirements for Class 1 and Class 10 buildings including concrete slab-on-ground construction.
- AS 1379 — Specification and supply of concrete. Covers ready-mix concrete ordering, strength testing and slump requirements used when ordering from a batch plant.
- AS 2870 — Residential slabs and footings. Soil classification system (Class A, S, M, H1, H2, E, P) that determines footing depth and slab thickness for reactive soils. Most residential concrete work references this standard.
- AS 3610 — Formwork for concrete. Relevant when forming slabs and footings on site, covering load requirements, release agents and stripping times.
Common concrete questions for Australian projects
How much does a cubic metre of concrete weigh?
Standard ready-mix concrete weighs approximately 2,400 kg/m³ (2.4 t/m³). Lightweight concrete can be as low as 1,600 kg/m³ and heavy aggregate mixes used for shielding can exceed 3,000 kg/m³. For most residential work — slabs, footings, paths — use 2,400 kg/m³ as a planning figure for transport and pour logistics.
What concrete strength (MPa) should I use for a residential slab in Australia?
AS 2870 and most Australian engineers specify N20 (20 MPa) as the minimum for residential slab-on-ground in Class A and S soil conditions. N25 is commonly specified for Class M and H sites or where corrosive soil conditions exist. Always follow your engineer's or certifier's specification — the NCC does not permit a structural engineer's spec to be substituted with a lower-grade mix.
How many 20 kg bags of concrete are in a cubic metre?
A 20 kg bag of premix concrete yields approximately 0.010 m³ (10 litres) when mixed. This means you need 100 bags per cubic metre. A 25 kg bag yields approximately 0.0125 m³, so you need 80 bags per m³. A 40 kg bag yields approximately 0.020 m³, so you need 50 bags per m³. For volumes above about 0.3 m³, ordering ready-mix from a batch plant is usually more economical than bagged product.
How much concrete coverage from a premix bag?
A 20 kg bag covers approximately 0.010 m³. At 100 mm slab depth, that covers 0.10 m². At 75 mm, it covers 0.133 m². Use the slab calculator above — enter your dimensions and it returns the exact bag count and volume so you can compare with ready-mix pricing from your local supplier (Boral, Holcim, Cockburn Cement etc.).
Do I need a building permit for a concrete slab in Australia?
It depends on state and purpose. Generally, a concrete slab that forms part of a building (e.g. a house slab or garage slab) requires a building permit. A small garden path or driveway apron in a residential property typically does not — but rules vary by local council and state. In Victoria, Queensland and NSW, most structural slabs require a building permit and engineer-certified footings design. Always check with your local council or a registered building surveyor before proceeding.