Work out exactly how much concrete you need for a slab, footing, post hole or set of steps — in cubic metres, premix bags, tonnes and dollars.
The short answer
Concrete volume is length × width × thickness, with every measurement in metres. A 4 m × 3 m slab at 100 mm is 4 × 3 × 0.1 = 1.2 m³. Add 5–10% for waste and order 1.3 m³.
That same slab is roughly 130 × 20 kg bags — which is why anything over about half a cubic metre is worth ordering ready-mix instead.
Pour 1
Advanced options
Trades allow 5–10%. Use 15% for bored holes in sand.
Yields vary by brand — check the bag.
Supply only. Excludes pump and short-load fees.
On this page
How to calculate concrete volume
Every concrete quantity comes down to one calculation: the area of the pour multiplied by its depth. The only thing that catches people out is units. Slab dimensions are usually in metres but thickness is quoted in millimetres, and if you multiply 4 × 3 × 100 you get 1,200 — a number that means nothing. Convert the thickness to metres first and you get the 1.2 m³ you actually need.
Volume (m³) = Length (m) × Width (m) × Thickness (m)
Thickness in metres = millimetres ÷ 1000
100 mm = 0.1 m · 125 mm = 0.125 m · 150 mm = 0.15 m
Worked example — 6 × 4 m shed slab at 100 mm
Batch plants sell in 0.1 m³ increments, so always round up to the next tenth.
Volume formulas for other shapes
Slabs are the easy case. These are the three other pours that come up constantly on a residential site.
Strip footings and trenches
A footing is just a long, narrow slab. Measure the total run of the trench — including the returns around corners — then multiply by width and depth. Measure the trench you actually dug, not the one on the plan. Excavators overcut, and a 300 mm trench that came out at 350 mm is 17% more concrete.
Volume = Total run (m) × Width (m) × Depth (m)
20 m run × 0.3 m wide × 0.4 m deep = 2.4 m³
Post holes and piers
Round holes use the circle formula. The mistake here is using the diameter where the formula wants the radius, which quadruples your answer.
Volume = π × (diameter ÷ 2)² × depth × number of holes
300 mm ⌀ × 600 mm deep
= 3.1416 × 0.15² × 0.6
= 0.042 m³ per hole — about 4 × 20 kg bags
Subtract the post itself if it’s a solid section in a tight hole, though for a standard 100 × 100 post in a 300 mm hole the difference is small enough that it just becomes your waste allowance.
Poured steps
Steps are a stack of rectangular prisms, each one carrying the treads above it. The bottom step is the full height of the flight; the top step is one riser deep. The calculator handles this automatically, but by hand you work out each step separately and add them.
Worked example — 4 steps, 1.2 m wide, 175 mm rise, 280 mm tread
How much waste should you allow?
Waste is not optional and it is not the same on every job. Running short mid-pour is the worst outcome on a concrete site — you get a cold joint where the first batch has stiffened before the second arrives, and that joint is a permanent weak line through the slab. Ordering 5% too much costs a few dollars. Ordering 5% too little can cost you the slab.
| Situation | Allow | Why |
|---|---|---|
| Slab on a formed, compacted base | 5% | Base is level, losses are minimal |
| Slab on a rough sub-base | 10% | Low spots swallow more than you expect |
| Strip footings in clay | 10–15% | Trench walls slump and overcut |
| Post holes bored in sand | 15% | Sides collapse and belly out below grade |
| Any pumped pour | +0.25 m³ | Concrete left in the line is unrecoverable |
How many bags of concrete per cubic metre?
Bag yield varies by brand, which is why bag counts on different websites disagree. A 20 kg bag of premix yields somewhere between 0.009 m³ and 0.010 m³ depending on how the manufacturer batches it, so a cubic metre takes anywhere from 100 to 116 bags. Check the number printed on the bag before you buy a pallet.
| Bag size | Yield | Per m³ | Best for |
|---|---|---|---|
| 20 kg | 0.009–0.010 m³ | 100–116 | Post holes, small repairs, mower strips |
| 25 kg | 0.0125 m³ | 80 | Small footings, garden edging |
| 40 kg | 0.020 m³ | 50 | Trade size — fewer trips, heavier lift |
| 60 lb | 0.45 ft³ | 60 / yd³ | Imperial equivalent of 25 kg |
| 80 lb | 0.60 ft³ | 45 / yd³ | Imperial equivalent of 40 kg |
Bagged or ready-mix? Where the line sits
Under 0.5 m³
Use bags
A batch plant charges a short-load fee of $150–$300 that destroys the per-metre advantage. Ten fence posts is 0.42 m³ — that’s a bag job.
0.5 – 1.2 m³
Genuinely arguable
Bags mean 50–130 of them and hours of mixing. Ready-mix means a short-load fee. A mini-mix supplier usually beats both — ring one first.
Over 1.2 m³
Order ready-mix
Bags run 3–4× the price per cubic metre, and hand-mixing that volume takes long enough that your first batch is going off before your last is placed.
Bagged premix: roughly $850–$1,300/m³ once you count the bags needed to make a full cubic metre. Prices vary by state and by distance from the batch plant — always get a quote for your actual postcode.
What thickness does your slab need?
Thickness drives volume more than anything else, so get it right before you calculate. These are common residential figures. Anything structural, and anything on reactive clay, needs an engineer’s design under AS 2870 — not a table on a website.
| Application | Thickness | Concrete per 10 m² |
|---|---|---|
| Garden path, mower strip | 75–85 mm | 0.75–0.85 m³ |
| Footpath, pedestrian only | 85–100 mm | 0.85–1.0 m³ |
| Shed or garage floor | 100 mm | 1.0 m³ |
| Residential driveway | 100–125 mm | 1.0–1.25 m³ |
| Truck or caravan access | 150 mm | 1.5 m³ |
| House slab on ground | 100 mm + beams | Engineer designed |
House slabs are the exception to everything above. A slab on ground is not a flat plate — it has thickened edge beams and internal beams whose depth is set by the site’s soil classification. On a Class H or E reactive clay site those beams can be 600 mm or deeper and can easily be half the total concrete volume. Calculate the flat plate and the beams as separate pours and add them.
Which concrete strength should you order?
Strength is quoted in MPa and describes compressive strength at 28 days. Ordering higher than you need wastes money; ordering lower than the application requires means the slab cracks or spalls.
| Grade | Typical use |
|---|---|
| N20 | Footings, post holes, non-structural fill, garden edging |
| N25 | Residential slabs, paths, patios, shed floors |
| N32 | Driveways, exposed slabs, anything taking vehicle load |
| N40 | Heavy-duty industrial floors, structural columns and beams |
Strength doesn’t change the volume you order, but it does change the price — expect roughly $15–$25 per cubic metre between each grade. You’ll also be asked about slump (how wet the mix is, typically 80–100 mm for a slab) and aggregate size (20 mm for slabs, 10 mm for thin sections and tight reinforcement).
How much does concrete weigh?
Wet concrete weighs approximately 2.4 tonnes per cubic metre. Cured concrete is slightly lighter at around 2.3 t/m³ once the mix water has been consumed and evaporated.
This matters for two reasons. Access: a 6 m³ agitator carries about 14.4 tonnes of concrete plus roughly 12 tonnes of vehicle, so a truck at capacity is close to 27 tonnes on your driveway or verge. Plenty of residential driveways, and most suburban footpath crossings, are not built for that. Suppliers will often send two half-loaded trucks or ask you to arrange a pump for exactly this reason.
Formwork: 2.4 tonnes per cubic metre is a lot of hydraulic pressure against your boards. A 400 mm deep footing exerts serious side load, and under-braced formwork blowing out mid-pour is the single most common way a DIY concrete job goes wrong.
Ordering — what the batch plant will ask
- Volume in cubic metres, to one decimal place. They batch in 0.1 m³ steps.
- Strength grade — N20, N25 or N32.
- Slump — 80–100 mm for a typical slab. Don’t add water on site; it reduces strength and voids the supplier’s warranty.
- Aggregate size — 20 mm standard, 10 mm for thin or heavily reinforced sections.
- Access — truck size, whether you need a mini-mix, and how far the chute has to reach. Chutes typically reach only 3–4 m from the truck.
- Placement method — direct chute, wheelbarrow, or pump.
Seven mistakes this calculator prevents
- Mixing units. Multiplying metres by millimetres gives a number 1,000× too large.
- Using diameter instead of radius in the circle formula — four times too much concrete on post holes.
- Forgetting waste entirely, then running short the moment the sub-base has a low spot.
- Confusing cement bags with concrete bags. Different products, wildly different quantities.
- Calculating a house slab as a flat plate. Edge and internal beams are often half the volume.
- Ignoring the pump line. A quarter of a cubic metre stays in the hose.
- Rounding down. The cost of extra is trivial next to the cost of a cold joint.
Frequently asked questions
How much concrete do I need for a 4 × 3 m slab?
At 100 mm thick, a 4 m × 3 m slab needs 1.2 m³ of concrete (4 × 3 × 0.1). Adding a 10% waste allowance brings that to 1.32 m³, so you’d order 1.4 m³. At 125 mm thick the same slab needs 1.5 m³ before waste.
How many 20 kg bags of concrete make a cubic metre?
Between 100 and 116 bags, depending on brand. Most Australian premix products yield 0.009–0.010 m³ per 20 kg bag. Check the figure printed on the bag — manufacturers state it, and the variation between brands is real.
How much concrete do I need per fence post?
A standard 300 mm diameter hole at 600 mm deep takes about 0.042 m³, which is roughly four 20 kg bags. For ten posts that’s 0.42 m³ or about 42 bags. Sandy ground where holes belly out can push this 15% higher.
Is it cheaper to mix your own concrete?
Buying cement, sand and aggregate separately is cheaper per cubic metre than bagged premix, but only worthwhile above about 0.5 m³ where you have a mixer and space to store materials. Above roughly 1.2 m³, ready-mix delivered beats both on cost and consistency.
How much does 1 m³ of concrete cost in Australia?
Ready-mix supply typically runs $250–$380 per cubic metre in metro areas, before pump hire, short-load fees or weekend rates. Bagged premix works out around $850–$1,300 per cubic metre. Prices vary significantly by state and by distance from the batch plant.
How much does a concrete truck hold?
A standard agitator truck carries up to 6 m³, with larger units up to 8 m³. Mini-mix trucks deliver from around 0.2 m³ up to 2 m³ and are the right choice for tight access or small pours. A fully loaded 6 m³ truck weighs close to 27 tonnes, which many residential driveways cannot take.
How thick should a concrete slab be?
100 mm suits shed floors and garage slabs, 100–125 mm suits residential driveways, and 150 mm is used where trucks or caravans will drive on it. Paths can go down to 75–85 mm. House slabs on ground are engineer-designed under AS 2870 and include thickened beams.
How long does concrete take to cure?
Concrete is generally walkable after 24–48 hours and can take light vehicle traffic at around 7 days, but it reaches its specified strength at 28 days. Keeping it damp for the first week matters more than most people realise — concrete that dries too fast is permanently weaker.
How much does concrete weigh?
About 2.4 tonnes per cubic metre wet, and around 2.3 tonnes per cubic metre once cured. A 6 m³ load is roughly 14.4 tonnes of concrete alone.
What is the difference between m² and m³ for concrete?
Square metres measure the surface area of your slab; cubic metres measure the volume of concrete required. Concrete is always ordered in cubic metres. Multiply the area in m² by the thickness in metres to convert: 24 m² at 100 mm is 24 × 0.1 = 2.4 m³.
Related calculators
Concrete Bags CalculatorConvert cubic metres to 20, 25 or 40 kg bags
Concrete Slab Cost CalculatorDelivered and laid price per square metre
Post Hole CalculatorBags per fence post and depth guidance
Cubic Metre CalculatorVolume in m³ for any material
Square Metre CalculatorWork out floor and wall area first
How we calculate this
Volume uses standard geometric formulas with all inputs normalised to metres before multiplication. Wet density is taken as 2.4 t/m³. Premix bag yields are 0.010 m³ for 20 kg, 0.0125 m³ for 25 kg and 0.020 m³ for 40 kg, at the upper end of the range Australian manufacturers publish — check your specific product, as some yield 0.009 m³. Truck capacity assumes a 6 m³ standard agitator. Cost figures are supply-only indicative rates for Australian metro areas and exclude pumping, short-load fees, out-of-hours rates and labour.
This calculator gives quantities, not structural design. Slab thickness, reinforcement and footing dimensions for any structural element should come from an engineer working to AS 3600 and AS 2870, particularly on reactive clay sites.