Concrete Calculator

How to Use the Concrete Calculator

Pick what you're pouring — a slab, a strip footing, round piers or a set of steps — and enter the sizes in metres and millimetres. The concrete calculator shows the volume in cubic metres, the volume with a wastage allowance, the ready-mix quantity to order and how many 20 kg or 30 kg premix bags you'd need, plus the approximate weight and, if you add prices, the cost of each option. For slabs it also estimates reinforcing mesh sheets and plastic membrane.

Whether you call it a concrete volume calculator, a concrete slab estimator or just a conc calc, the maths is the same: get the volume right first, then decide how to buy it.

Concrete Volume Formulas

Worked Example: a 6 × 4 m Slab

A shed slab measures 6 m × 4 m and is 100 mm thick. The net volume is 6 × 4 × 0.1 = 2.40 m³. Adding 10% for waste brings it to 2.64 m³, so you'd order 2.8 m³ of ready-mix (the next 0.2 m³ step). Mixing it yourself would take 278 × 20 kg bags (or 186 × 30 kg bags) at typical yields — well past the 60-bag rule of thumb, so ready-mix is the practical choice. That much concrete weighs about 6.34 tonnes.

The slab covers 24 m², which needs about 2 sheets of 6.0 × 2.4 m reinforcing mesh once 200 mm laps are allowed for, and 26.4 m² of plastic membrane including 10% for laps and edges.

Concrete Strength Grades in Australia

Ready-mix in Australia is usually ordered by strength grade, such as N20, N25 or N32 — the number is the characteristic compressive strength in megapascals (MPa) at 28 days. N20 is commonly used for general DIY work such as paths and small pads, while N25 and above are commonly specified for driveways and structural work. If your job has an engineer's design, order the grade, slump and any additives it specifies; if not, tell your supplier what you're pouring and ask what they recommend.

Mesh, Membrane and Curing

Most slabs on the ground sit on a plastic membrane (vapour barrier) laid over a compacted base, with steel reinforcing mesh held up on bar chairs so it ends up within the slab rather than on the ground. The mesh type, cover and any extra bars for a driveway or house slab come from the design. Once poured, concrete gains strength over weeks, not hours: it's generally firm enough to walk on after about 24–48 hours, but it keeps strengthening and is usually tested at 28 days. Keeping the surface damp or covered for the first several days helps it cure properly and reduces cracking, particularly in hot or windy weather.

Frequently Asked Questions

What is the calculation for concrete volume?

Concrete volume is length × width × thickness, with every measurement in metres, giving cubic metres (m³). Thickness is usually quoted in millimetres, so divide it by 1,000 first: a 6 m × 4 m slab at 100 mm thick is 6 × 4 × 0.1 = 2.4 m³. For a round pier use π × radius² × depth, and multiply by the number of identical pours.

How do I use a concrete calculator in metres?

Enter lengths and widths in metres and slab thickness, footing sizes and pier diameters in millimetres — the way Australian plans and suppliers quote them. The calculator converts millimetres to metres for you and returns the volume in cubic metres, which is the unit ready-mix is sold in across Australia.

How much concrete do I need for a slab in Australia?

Work out the net volume (length × width × thickness), then add a wastage allowance — 10% is a common figure for spillage, an uneven sub-base and over-excavation. Ready-mix suppliers typically sell in 0.2 m³ steps, so round the total up to the next step. A 6 × 4 m slab at 100 mm needs 2.4 m³ net, 2.64 m³ with 10% waste, and an order of 2.8 m³.

How thick should a concrete slab or floor be?

As a general guide, garden paths are often around 75–100 mm, patios and small shed slabs around 100 mm, and driveways thicker (commonly 100–150 mm, with reinforcement) because they carry vehicles. House slabs and other structural floors are a different matter: they need an engineered design to AS 2870 for your site's soil class, so use your engineer's thickness and edge-beam details rather than a rule of thumb.

How many bags of concrete are in a cubic metre?

It depends on the product's yield. At a typical yield of about 0.0095 m³ per 20 kg bag, one cubic metre takes roughly 106 bags; at about 0.0142 m³ per 30 kg bag it's roughly 71 bags. Yields vary between brands and mixes, so check the bag and enter its figure in the yield field for an exact count.

Should I use premix bags or ready-mix concrete?

Bags suit small jobs like post holes, small pads and repairs, where you can mix and place each batch before the last one sets. Once a job needs more than about 60 bags it becomes heavy, slow work and hard to pour as one continuous slab, so ready-mix is usually the better choice. Small ready-mix loads often attract short-load or delivery fees, so compare full quotes — the calculator shows both costs when you enter prices.

How much extra concrete should I order for waste?

Allowing 5–10% is common for formed slabs on a well-prepared base, and more for footings and piers dug into soil, where trench sides and hole bottoms are rarely perfectly square. Running short mid-pour is far worse than a little left over — a second small delivery can cost more than the extra concrete and may leave a cold joint in the slab.

How do I estimate concrete for round piers or post holes?

Choose the round pier shape and enter the hole diameter in millimetres, the depth in metres and the number of holes. Each pier is π × (diameter ÷ 2)² × depth: six 450 mm piers 0.9 m deep come to about 0.859 m³ before waste. Small pier jobs are a common case where premix bags make more sense than a ready-mix truck.

Is a concrete estimator accurate enough to order from?

It's accurate for the dimensions you give it, so measure the actual formwork or excavation rather than the plan size — a slab that's 10 mm deeper than planned over a large area adds up quickly. Estimating concrete for a structural job should still be checked against your engineer's drawings, and your supplier can confirm the final order quantity.