Calculate concrete volume, bags, ready-mix quantity and cost for slabs, footings, walls, columns, stairs and circular pads.
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| Item | Value |
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Concrete is the most widely used construction material in Australia. Standard residential slab thickness is 100mm (10cm). Driveways typically use 100–125mm. Footings are usually 200–300mm deep and 400–600mm wide depending on soil and load.
| Bag size | Volume (wet) | Coverage (100mm slab) |
|---|---|---|
| 20kg bag | ~0.010 m³ | ~0.10 m² at 100mm |
| 25kg bag | ~0.012 m³ | ~0.12 m² |
| 40kg bag | ~0.020 m³ | ~0.20 m² |
For volumes over 0.5 m³, ordering ready-mix concrete (delivered by truck) is usually more economical. In Australia, a standard concrete truck carries 6–8 m³. Most suppliers have a minimum order of 0.2–0.5 m³ with a short-load surcharge.
Written and reviewed by Mohsin Iqbal · Last reviewed August 2026
Concrete is a mix of cement, sand, aggregate (gravel or crushed stone) and water that hardens into a solid mass. When you order it, you're not ordering a size — you're ordering a volume, almost always in cubic metres. A slab isn't "20 square metres of concrete", it's however many cubic metres that area works out to once you multiply by thickness.
This is the single most common source of a wrong concrete order: quoting area instead of volume. A 20 m² slab at 100 mm thick is 2 m³. The same 20 m² at 150 mm is 3 m³ — 50% more concrete for the same footprint, just because the thickness changed.
Every shape reduces to the same idea: multiply the dimensions together to get a volume, then add a waste allowance.
Volume = Length × Width × Thickness (for a slab, footing or wall)
Volume = π × radius² × thickness (for anything circular)
The complication is unit mixing. Slab length and width are usually in metres, but thickness is almost always specified in millimetres — a 100 mm slab, not a 0.1 m slab. Forgetting to convert is the second most common ordering mistake, and it's a big one: entering 100 instead of 0.1 inflates the volume by 1,000×.
| Step | Working | Result |
|---|---|---|
| Slab size | 3m × 3m | 9 m² |
| Thickness | 100mm = 0.1m | — |
| Volume | 3 × 3 × 0.1 | 0.9 m³ |
| +10% waste | 0.9 × 1.1 | 0.99 m³ |
| 20kg bags needed | 0.99 ÷ 0.01 | ~99 bags (100 rounded) |
These are all the same calculation — a flat rectangular pour, differing only in what sits on top. Thickness is the variable that changes most between them.
| Use | Typical thickness |
|---|---|
| Garden path, stepping area | 75–100mm |
| Residential patio | 100mm |
| Standard driveway (light vehicles) | 100–125mm |
| Driveway carrying trucks or heavy vehicles | 150–200mm |
| House slab (residential) | 100–150mm, over a reinforced footing system |
A house slab isn't just a flat pour — it's normally engineered with edge beams and internal ribs sized to the soil classification, which is well beyond what a general calculator like this one should estimate. For anything structural, get a slab design from an engineer.
A footing is a narrow trench of concrete that transfers load from a wall or structure into the ground below. The calculation is the same length × width × depth formula, just with much smaller width and depth numbers than a slab.
| Footing type | Typical width | Typical depth |
|---|---|---|
| Light garden wall / fence | 250–300mm | 200–300mm |
| Residential strip footing | 350–450mm | 300–450mm |
| Load-bearing wall footing | 450–600mm | 450–600mm |
Footing sizes depend on soil type, wall load and local council requirements — the figures above are typical starting points, not a substitute for an engineer's specification on anything structural.
Circular vertical elements — deck posts, verandah supports, fence post footings — use the cylinder formula: π × radius² × height. Multiply by the number of identical columns to get the total.
A common mistake here is entering diameter where the formula needs radius, or vice versa. This calculator asks for diameter and halves it internally, which matches how most people measure a post hole with a tape.
A concrete wall (retaining wall, boundary wall) uses length × height × thickness — effectively a slab standing on its edge. Retaining walls in particular need engineering input for reinforcement and drainage; this calculator gives you the concrete volume only, not a structural design.
Stairs are the shape people get wrong most often, because a staircase isn't a simple block. Each step needs solid concrete filling all the way down to the ground beneath it, not just its own tread depth — so the bottom step effectively carries the full height of the staircase above it, layered like a stack of rectangles.
Stair volume = Width × Run × Rise × N(N+1) ÷ 2
where N is the number of steps
| Step | Working | Result |
|---|---|---|
| 4 steps, 1.0m wide, 175mm rise, 280mm run | 1.0 × 0.280 × 0.175 × (4×5)/2 | — |
| N(N+1)/2 | (4×5)/2 | 10 |
| Volume | 1.0 × 0.280 × 0.175 × 10 | 0.49 m³ |
| +10% waste | 0.49 × 1.1 | 0.539 m³ |
Round pads — under a water tank, a spa, a circular garden feature — use π × radius² × thickness. As with columns, enter the diameter you'd measure across the middle; the calculator halves it for you.
| Bagged premix | Ready-mix truck | |
|---|---|---|
| Best for | Small jobs under ~0.5–1 m³ | Anything larger |
| Cost per m³ | Roughly 3–4× more expensive | Cheaper per m³, but has delivery minimums |
| Labour | Mix by hand or mixer, bag by bag | Delivered ready to pour |
| Consistency | Depends on careful mixing | Batched to a specified strength |
The crossover point is usually around 1 m³. Below that, bags are often more practical even though they cost more per cubic metre, because most ready-mix suppliers have a minimum order and a small-load surcharge that can make a tiny delivery uneconomical.
| Bag size | Approx. yield | Bags per m³ |
|---|---|---|
| 20kg | ~0.010 m³ | ~100 bags |
| 25kg | ~0.0125 m³ | ~80 bags |
| 40kg | ~0.020 m³ | ~50 bags |
These figures vary by brand, typically within about ±10% — check the yield printed on your specific bag if you're ordering close to the exact amount. Buy a few extra bags rather than running short mid-pour; a cold joint from stopping and restarting a pour partway through is a real weak point in the finished concrete.
In the US, premix concrete is sold in 40lb, 60lb and 80lb bags rather than the 20kg/25kg/40kg sizes used in Australia, and ready-mix is ordered and priced by the cubic yard instead of the cubic metre. This calculator gives you both — select feet or yards as your unit above, and the results table shows 80lb, 60lb and 40lb bag counts alongside the AU sizes.
| Bag size (US) | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | ~0.3 cu ft | ~90 bags |
| 60 lb | ~0.45 cu ft | ~60 bags |
| 80 lb | ~0.6 cu ft | ~45 bags |
US ready-mix concrete typically costs $125–$180 per cubic yard delivered, depending on region and mix design, with most suppliers charging a short-load fee for orders under about 3–4 cubic yards — noticeably higher than Australia's typical minimum, since concrete trucks in the US commonly carry 8–10 cubic yards.
The US rates concrete strength in pounds per square inch (psi), while Australia uses megapascals (MPa) — the N-grade numbers (N20, N25, N32, N40) are the MPa figure itself. Both scales measure the same thing: the compressive strength concrete reaches at 28 days.
| Australian grade | Approx. US equivalent | Typical use |
|---|---|---|
| N20 (20 MPa) | ~2,900 psi | Paths, non-structural fill |
| N25 (25 MPa) | ~3,600 psi | House slabs, driveways, patios (matches common US 3,000–4,000 psi residential mix) |
| N32 (32 MPa) | ~4,600 psi | Footings, structural slabs |
| N40 (40 MPa) | ~5,800 psi | Columns, engineered elements |
A standard US residential driveway or sidewalk pour commonly uses 3,000–4,000 psi mix — roughly comparable to Australia's N25 — while structural footings often step up to 4,000 psi or higher, similar to N32.
The waste allowance covers ground that isn't perfectly flat, formwork that isn't perfectly square, spillage, and the reality that suppliers round up to the nearest available load. 10% is the standard default for straightforward slabs and footings. Stairs and other complex shapes warrant 15%, since irregular formwork loses more to gaps and corners. A very simple, well-formed pour on flat ground can sometimes get away with 5%.
Ready-mix pricing in Australia is typically $250–$380 per m³ depending on region, strength grade and access, plus a delivery fee and, if needed, a pump hire fee for pours the truck chute can't reach directly. Bagged concrete runs roughly $850–$1,300 per effective m³ once you total the bag cost — noticeably more per cubic metre, which is the trade-off for not needing a minimum truck order.
This calculator's cost figures are estimates based on the prices you enter — get an actual quote from a local supplier before ordering, since prices vary significantly by region and by how difficult your site is to access.
| Stage | Typical timing |
|---|---|
| Walkable | 24–48 hours |
| Light vehicle traffic | ~7 days |
| Full design strength | 28 days |
Concrete continues gaining strength well past the point it looks and feels solid — the 28-day figure is an industry standard reference point, not the moment it becomes usable.
| Cubic metres | Cubic yards | Cubic feet | Litres |
|---|---|---|---|
| 0.5 | 0.654 | 17.66 | 500 |
| 1 | 1.308 | 35.31 | 1,000 |
| 2 | 2.616 | 70.63 | 2,000 |
| 5 | 6.540 | 176.6 | 5,000 |
| 10 | 13.08 | 353.1 | 10,000 |
| Truck size | Typical capacity |
|---|---|
| Mini mixer | 1–2 m³ |
| Standard agitator truck | 6–8 m³ |
| Large agitator truck | Up to 9–10 m³ |
Small mini-mixers can access tighter residential sites where a full-size truck can't, often at a higher rate per cubic metre.
Standard concrete weighs approximately 2,400 kg per cubic metre. This calculator uses that figure for its weight estimate — useful for checking whether a suspended slab or structure can bear the load, though any load-bearing calculation should be confirmed by an engineer.
Multiply length by width by thickness (in metres) to get cubic metres, then add 10% waste allowance. For a 3m x 3m slab at 100mm thick: 3 x 3 x 0.1 = 0.9 m³, plus 10% waste = 0.99 m³. Enter your dimensions above for an exact figure across any shape.
Roughly 100 bags of 20kg, 80 bags of 25kg, or 50 bags of 40kg premix — figures vary by brand by around ±10%, so check the yield printed on your specific bag for anything precise.
100–125mm for standard residential driveways carrying cars. Driveways that need to carry trucks, caravans or heavy vehicles should go to 150–200mm, ideally with engineering input on reinforcement.
A cube one metre on each side — 1,000 litres, or about 2,400kg of standard concrete. It's the standard unit ready-mix suppliers price and deliver by.
10% is the standard default for straightforward slabs and footings. Increase to 15% for stairs and other complex shapes, which lose more to irregular formwork and corners. A very simple pour on flat, well-prepared ground can sometimes get away with 5%.
Ready-mix is cheaper per cubic metre and is the practical choice above about 1 m³, but most suppliers have a delivery minimum. Bagged concrete costs roughly 3–4 times more per cubic metre but has no minimum order, making it the practical choice for smaller jobs like post holes or small patches.
Yes, ready-mix suppliers can usually deliver fractional cubic metres, but many apply a minimum order or small-load surcharge, often around 1 m³. For anything smaller, bagged concrete is often more economical once that surcharge is factored in.
1,000 litres. A cubic metre is defined as 1m x 1m x 1m, and one litre is one thousandth of a cubic metre by definition.
Standard concrete weighs approximately 2,400kg per cubic metre once cured. A 1 m³ pour therefore weighs roughly 2.4 tonnes — a figure worth checking against load ratings for anything on a suspended slab or structure.
N20 suits paths and non-structural fill, N25 is standard for house slabs, driveways and patios, N32 suits footings and structural slabs, and N40 is for columns and engineered structural elements. Confirm the required grade with your supplier or engineer for anything load-bearing.
Small non-structural jobs like a garden path or post holes are commonly DIY. Anything structural — a house slab, a load-bearing footing, a retaining wall — should involve a qualified tradesperson or engineer, and may require council approval depending on your location and the scope of work.
Walkable in 24–48 hours, able to handle light vehicle traffic around 7 days, and reaching its full design strength at 28 days — the industry-standard reference point used for structural calculations, even though the concrete is usable well before then.
Multiply your required volume in cubic metres by the ready-mix price per m³ (typically $250–$380 in Australia), then add delivery and, if needed, pump hire fees. For bagged concrete, multiply the number of bags by the price per bag — this calculator does both automatically from your entered prices.
Real sites are never perfectly flat or perfectly formed. The waste allowance covers ground irregularity, formwork gaps, spillage during placement, and the practical reality that a supplier delivers in rounded increments. Skipping it is the most common reason a pour runs short.
About 45 bags of 80lb premix, 60 bags of 60lb, or 90 bags of 40lb per cubic yard. This calculator shows all three US bag sizes alongside the AU 20kg/25kg/40kg sizes in the results table — just switch the unit selector to feet or yards.
Multiply length by width by thickness to get volume — in the US this is usually expressed in cubic yards, so convert feet to yards or let this calculator do it for you. A 10ft × 10ft slab at 4in thick works out to about 1.23 cubic yards before waste.
3,000–4,000 psi is standard for US residential driveways, patios and sidewalks — roughly equivalent to Australia's N25 grade. Structural footings and anything load-bearing often step up to 4,000 psi or higher. Confirm the required psi with your local building code or engineer for anything structural.