Concrete Calculator Australia

From dimensions to a batched truck.

Volume is length × width × thickness, returned in cubic metres. Round columns use π × radius² × height. Stairs are figured riser-by-riser as a stack of rectangles — the correct method for cast-in-place steps.

A waste allowance of 5% to 10% is added to the raw volume. Eight per cent suits most residential work — slabs, paths, garage pads. Push it to ten for hand-dug footings or rough excavation. Batching plants supply in 0.2 m³ increments for agitator trucks; the calculator rounds up to the next quarter metre to give you a clean ordering figure.

Bag yields use the printed values on Australian pre-mixed concrete (Boral, Hanson, Cement Australia): a 20 kg bag gives about 0.010 m³, a 30 kg bag about 0.015 m³. The reinforcement estimate uses N12 deformed bar at 300 mm centres each way — typical for a residential slab to AS 3600 exposure class A1 or A2.

Choosing the right strength.

Australian concrete is specified by characteristic compressive strength at 28 days, in megapascals. AS 1379 defines two categories: Normal class (N) with grades N20 to N50, and Special class (S) for performance-specified mixes. The exposure classifications come from AS 3600 and drive the minimum grade and cover.

Exposure class drives durability across Australia. A1 and A2 cover most inland residential work. B1 and B2 apply within 1 km of surf coast or 100 m of still saltwater. C1 and C2 are for direct salt spray or tidal zones. The further east, north, or coastal you build, the harder the AS 3600 cover requirement bites.

Residential slabs under AS 2870.

For most Australian homes, the slab and footings are designed to AS 2870 — the residential slabs and footings standard. Site classification by soil reactivity is the starting point: Class A (stable rock), Class S (slightly reactive), Class M (moderately reactive), Class H1/H2 (highly reactive), Class E (extremely reactive), Class P (problem site).

Reactive clay sites — common across Adelaide, Melbourne, Sydney's south-west, and South-East Queensland — need engineered footings. A Class H1 or H2 site will typically demand a stiffened raft or waffle slab with 600–800 mm deep edge beams. A Class P site (problem site) requires individual engineering by a CPEng. Skipping site classification leads to seasonal cracking from clay shrink-swell — by far the most common cause of slab failure in Australian housing.

Heat, sun, and curing.

Cold-weather concreting isn't the main concern across most of Australia — heat is. Above 32°C, surface evaporation outpaces bleed water, and plastic shrinkage cracking begins almost immediately. AS 3600 references practices from the Cement Concrete & Aggregates Australia (CCAA) for hot-weather placement, but the principles are simple:

• Pour early morning in summer (target before 10 am)
• Shade fresh concrete with sails or hessian within minutes of finishing
• Apply evaporative retarder or aliphatic alcohol before screeding
• Keep curing membrane in place 7 days minimum; longer in WA and NT summers
• Reject concrete delivered above 35°C (AS 1379 limit for normal-class)

Workability is specified by slump in millimetres at the truck (AS 1012.3.1). Residential flatwork is typically 80 mm; suspended slabs and pump work run 100–120 mm. Higher slump pours easier but loses strength if added water is the cause — ask for a superplasticiser admixture instead. Concrete placed and protected properly hits its specified strength at 28 days through hydration, not drying.

For coastal sites (B2, C1, C2 exposure), AS 3600 cover requirements increase to protect reinforcement from chloride-induced corrosion. Coastal NSW, the Sunshine Coast, Perth's beach suburbs — these all need N32 minimum and 40–50 mm cover. Inland Canberra or Bendigo can get away with N25 and 30 mm cover at A2 exposure.

Quick answers.

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