Concrete calculator

How much concrete: the formulas

Concrete is ordered by the cubic metre, so every job comes down to a volume. Work in metres. A thickness of 100 mm is 0.1 m, and 75 mm is 0.075 m.

The steps formula counts each step as a solid block sitting on the ones below. With three steps, the bottom one is one rise high, the second two and the top three, so the total is 1 + 2 + 3 = 6 rise-and-going blocks. Three steps 1.2 m wide with a 170 mm rise and a 300 mm tread take 1.2 × 0.17 × 0.3 × 6 = 0.37 m³. If you are building steps on an existing slope, subtract the ground under them, or measure the actual form and use the slab setting for each part.

Allow 10 % extra for small jobs. The Newcastle supplier Hunter Readymixed recommends this for spillage and uneven ground, then rounding up to the next 0.1 m³. Their example: a garage floor measured at 2.85 m³ becomes 3.13 m³ with the allowance, so you order 3.2 m³. Running short costs more than a little surplus, because a second small delivery pays its own fees.

How thick should the concrete be?

Cement Concrete & Aggregates Australia (CCAA), the industry body, publishes minimum thicknesses and matching reinforcement for residential paving in its data sheet on driveways and paths:

Normal-class N20 concrete suits most home paving, including typical driveways. The CCAA recommends N25 for the 3–10 tonne case and stronger mixes for saline soils. Anything between the kerb and your boundary, such as a crossover or the footpath, has to meet your council's specification for thickness, reinforcement and strength, so check that before you order.

Bags or ready-mix?

A 20 kg bag makes only about 9–10 litres of concrete, so 100–110 bags make a cubic metre. The calculator uses the yield each maker states:

Ready-mix costs far less per cubic metre. The online ordering service MixHub put Sydney N25 at $340–370 per m³ in 2026 before fees, while a 20 kg bag costs $8–12, or roughly $900–1,300 per m³. Small ready-mix orders pay extra, though. Holcim's published fees for the NSW North Coast charge $65 plus GST for every cubic metre a load falls short of 4 m³. A 0.7 m³ order is 3.3 m³ short, which adds about $236 including GST.

With the example prices in the calculator, bags win below about a third of a cubic metre, around 35–40 bags. Ready-mix wins above that, even with the short-load fee. Other things push toward ready-mix well before the costs cross:

• Weight and mixing. Seventy-three 20 kg bags weigh 1.46 tonnes: about three loads in a typical 6 × 4 box trailer, which can legally carry about 500 kg (see the trailer load calculator). Every bag is then mixed by hand or in a small mixer.
• One continuous pour. Bags mixed a few at a time can leave visible joints between batches. The CCAA advises placing concrete continuously from one corner, with new concrete placed against the face of what is already down.
• Placing time. The CCAA puts wheelbarrow placing at about 1–1.5 m³ an hour over no more than about 50 m, so a 0.7 m³ truck is a comfortable job for two people.

Bags make sense for post holes, pier footings and small pads, where a few bags are mixed at a time. The post hole concrete calculator handles fence posts, with the post and the drainage gravel taken out of the hole.

Mesh, bar chairs and joints

Mesh does not make a slab stronger or let it carry more load. The CCAA is explicit that its job is to hold tightly closed any cracks that form. The calculator counts what a simple rectangular slab needs:

• Mesh sheets. SL mesh comes in 6.0 × 2.4 m sheets. Sheets overlap by two cross wires, about 225 mm. The count assumes offcuts are reused; for awkward shapes, add one.
• Bar chairs. The mesh sits in the top half of the slab with at least 30 mm of concrete over it, held up on bar chairs at most 800 mm apart. Light meshes such as SL62 need them closer. Never walk the mesh down into the wet concrete or hook it up afterwards.
• Control joints. These are grooves or saw cuts that make the concrete crack in a straight line where you want it. Space them at no more than the distance in the table: 3 m for a 100 mm slab. Keep panels roughly square, never longer than 1.5 times their width. Add a joint at every change of shape and wherever the slab meets the house or another slab (an isolation joint).
• Edge formwork. The perimeter of the slab, in timber boards or steel edge forms, set to the finished levels.

Before you pour

1. Base. Strip all topsoil, dig out soft spots and replace them with compacted road base. On reactive clay, the CCAA recommends a non-reactive sub-base, 100 mm in most cases. Plastic sheeting or 20 mm of clean sand under the slab more than halves the friction that makes slabs crack.
2. Fall. Give the surface a fall of at least 1 in 100, which is 10 mm per metre, away from the house. Under the National Construction Code, paving that slopes away from the house must sit at least 50 mm below the house slab. That becomes 100 mm in low-rainfall areas or on sandy, well-drained soil. The damp-proof course must be at least 75 mm above the paving.
3. Weather. Do not pour in air above 30 °C or below 10 °C without precautions, and not at all if the concrete itself is above 35 °C or below 5 °C.
4. Curing. Keep the concrete continuously moist or covered for at least 3 days, or 7 days within 1 km of the coast or under heavier traffic. Wetting it morning and afternoon is not enough. In hot weather, do not cover it with black plastic.

A slab usually replaces lawn or garden. The paver calculator covers the paved alternative, and the stepping stone calculator the lightest option of all.

Concrete questions