Short answer
A 4 x 3 m patio slab at 100 mm thick needs 1.20 m³ of concrete, or about 1.32 m³ with 10% spare. That is roughly 143 bags of 20 kg mix (about 108 bags make a cubic metre) or one ready-mix delivery of 1.4 m³, ordered in 0.2 m³ steps. Ready-mix is usually the better choice above about 1 m³. Use the calculator for slabs, paths, round pads, strip footings and post holes.
Use this free concrete calculator to work out how much concrete you need for a slab, path, driveway, round pad, strip footing or set of post holes. It shows the volume in cubic metres, the number of 20 kg bags, the ready-mix to order, and indicative costs, plus the mesh and base for slabs.
Concrete calculator
Quantities are calculated exactly from the sizes you enter. Prices are indicative Australian ranges and vary by city, supplier and delivery. Ranges reviewed October 2026.
How to calculate how much concrete you need
Concrete is ordered by volume in cubic metres. Work out the volume of each pour, add a spare allowance, then round up to the amount you will order.
- Slab, path or pad: length (m) x width (m) x thickness (m). A 100 mm slab is 0.1 m thick.
- Round pad: 3.14 x (diameter ÷ 2) x (diameter ÷ 2) x thickness (m).
- Strip footing or trench: length (m) x width (m) x depth (m).
- Post holes: number of holes x 3.14 x (hole radius in m) x (hole radius in m) x depth (m).
Then add about 10% for spillage, uneven ground and over-excavated trenches. The Cement Concrete and Aggregates Australia guidance is to order roughly 10% extra, in 0.2 m³ steps, and notes that without planning up to 15% can be lost.
| Example | Volume | With 10% spare | 20 kg bags | Ready-mix order |
|---|---|---|---|---|
| Patio 4 x 3 m, 100 mm | 1.20 m³ | 1.32 m³ | 143 | 1.4 m³ |
| Driveway 6 x 3 m, 125 mm | 2.25 m³ | 2.48 m³ | 268 | 2.6 m³ |
| Strip footing 10 m x 300 x 400 mm | 1.20 m³ | 1.32 m³ | 143 | 1.4 m³ |
| Six post holes, 300 mm wide x 600 mm deep | 0.25 m³ | 0.28 m³ | 31 | 0.4 m³ |
One cubic metre weighs about 2.4 tonnes, and about 105 to 110 bags of 20 kg mix make a cubic metre (the common bag yields are around 9.3 to 9.4 litres), so the calculator uses 108.
How thick should a concrete slab be?
Thickness and strength depend on what the slab carries. These are the minimums in AS 3727.1 for residential pavements, as summarised by the HIA’s summary of AS 3727.1, and councils often ask for more under driveways.
| Use | Typical thickness | Strength grade |
|---|---|---|
| Footpath or garden path (foot traffic only) | 75 to 100 mm (75 mm minimum) | N20 |
| Patio or alfresco | 100 mm | N20 to N25 |
| Driveway for cars and light vehicles | 100 mm minimum; 125 mm is common | N25 |
| Driveway for trucks or heavy vehicles under 10 tonnes | 150 mm minimum | N32 |
| Shed or garage slab | 100 to 125 mm | N25 |
Council requirements can be stricter than the standard. For example, the Brisbane City Council driveway standards specify a 125 mm N25 slab for a driveway. The standard applies to stable sites and is guidance only (it is not referenced in the National Construction Code), so get engineering advice for anything outside it.
House slabs and footings are different. They are designed to AS 2870 for the soil classification of your site and need an engineer, as does any site with reactive or unstable soil, or loads over 10 tonnes.
Bags or ready-mix concrete?
A 20 kg bag of concrete mix costs about $9 to $14 at Bunnings in 2026, which works out at roughly $1,000 to $1,500 per cubic metre once you buy about 108 bags, before hiring a mixer or paying for your own labour. Delivered ready-mix is typically quoted at about $320 to $420 per cubic metre for N25 including GST, but suppliers do not publish retail prices, so ask for a quote for your postcode.
- Short-load fees: suppliers charge extra for small loads. For example, Holcim’s Sydney service fees (August 2026) list a short-load fee of $80 per cubic metre short of 4 m³ (ex GST), and other suppliers set minimum loads of about 3 to 4 m³. On a 1 m³ order that works out at about $240 (ex GST) at the Holcim Sydney rate.
- Waiting time: Holcim Sydney allows 30 minutes free, then charges $3.50 per minute (ex GST), so have the forms, base and helpers ready before the truck arrives.
- Pump hire: if the truck cannot reach, pump hire for a small residential job is commonly quoted at about $500 to $1,000.
Supplier guides suggest bags make sense for small jobs under roughly 0.5 to 0.8 m³, and ready-mix is usually quicker and cheaper above about 1 m³, because mixing 100 bags by hand takes hours and the pour has to be continuous for a consistent finish.
What does concreting cost installed?
If you hire a concreter, the price covers forming, base, mesh, concrete, placing and finishing. These are indicative installed rates per square metre including GST for a plain broom or float finish, compiled from published Australian cost guides in 2026. Nearly all published concreting prices come from contractors and lead-generation sites, which tend to run high, and there is no free HIA or Rawlinsons rate to check them against, so treat them as a guide and get written quotes.
| Job | Indicative installed cost per m² |
|---|---|
| Plain path | $75 to $150 |
| Patio or alfresco slab | $85 to $150 |
| Driveway | $75 to $130 |
| Exposed aggregate finish | $110 to $200 |
| Stamped concrete | $120 to $200 |
| Coloured concrete (extra over plain) | add $15 to $25 |
| Removing an old slab | $35 to $80 |
Larger or awkward jobs, poor access, steep sites, extra base depth and pump hire push prices higher. Compare quotes with the quote comparator, and check landscaping labour on the landscaper rates page.
Rules, approvals and safety
- NSW: paths, paving and driveways can be exempt development if they meet conditions, including a limit on total paved area (the lesser of 15% of the lot or 150 m²), minimum landscaped area, stormwater drainage and cut and fill under 600 mm. A driveway crossing still needs the roads authority’s written consent. See the NSW Planning exempt development rules for paving, and note that exclusions apply on some heritage, flood and bushfire land.
- Victoria and Queensland: councils control driveway crossovers and any work in the road reserve, and planning rules can still apply to paving on your land. Brisbane, for example, requires a driveway permit and sets gradient and spacing rules. Check with your council before you start.
- Silica dust: cutting or grinding hardened concrete releases respirable crystalline silica. Use wet cutting, on-tool dust extraction and a suitable respirator. See the SafeWork NSW silica checklist.
- Wet cement burns skin: wear gloves, eye protection and boots, and wash splashes off straight away.
Curing and common mistakes
Keep new concrete moist for at least 3 days, and 7 days is better, by covering it or misting it. Protect it from hot sun and wind, and avoid pouring on frozen ground.
- Ordering the exact volume. Uneven ground and trenches take more concrete than the sums say. Order about 10% extra in 0.2 m³ steps.
- Using the wrong grade or thickness. A path-grade slab under a car will crack, so match the grade to the load.
- Poor base preparation. A soft or uneven base causes cracking and uneven thickness. Compact the base and set the forms to level before the truck arrives.
- Mesh sitting on the ground. Reinforcement should sit on chairs in the top half of the slab with at least 30 mm of cover, not on the base.
- No joints. Concrete shrinks as it cures, so ask your concreter where control joints go so cracks form along them.
- Not planning access. Check the truck can reach, and book a pump early if it cannot.
Frequently asked questions
How many bags of concrete make one cubic metre?
About 105 to 110 bags of 20 kg mix make a cubic metre, because each bag yields roughly 9.3 to 9.4 litres. The calculator uses 108 bags per cubic metre, so a typical 100 mm path or patio of 10 m² (1 m³) needs about 108 bags before spare.
How much concrete do I need for a 3 x 3 m slab?
A 3 x 3 m slab at 100 mm thick is 0.90 m³. With 10% spare, order about 0.99 m³, which is roughly 107 bags of 20 kg mix or 1.0 m³ of ready-mix.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes. That matters for access, wheelbarrow loads and for checking that a suspended or retaining structure can carry the weight.
What do N20, N25 and N32 mean?
They are strength grades in megapascals: N20 is 20 MPa, N25 is 25 MPa and N32 is 32 MPa. AS 3727.1, as summarised by the HIA, calls for N20 and at least 75 mm for pedestrian paths, N25 and at least 100 mm where light vehicles drive, and N32 and at least 150 mm for commercial vehicles under 10 tonnes.
Do I need reinforcing mesh?
Pedestrian paths usually do not need it. Slabs that carry light vehicles commonly use SL72 mesh, and heavier slabs use SL82, held on bar chairs in the top half of the slab with at least 30 mm of cover. House slabs and footings are engineered, so follow the engineer’s design.
How long does concrete need to cure?
Keep it moist for at least 3 days, and 7 days is better, according to the Cement Concrete and Aggregates Australia guidance and the HIA’s summary of AS 3727.1. Protect it from hot sun and wind, and avoid pouring on frozen ground.
Do I need council approval for a concrete path or driveway?
It depends on where you live. In NSW, paths, paving and driveways can be exempt development when they meet conditions such as a limit on total paved area, drainage and cut and fill limits, and a driveway crossing still needs the roads authority’s consent. Victoria and Queensland councils generally control crossovers and road-reserve work, so check with your council before you pour.
Related tools
- Materials calculator - quantities for floors, paint and tiles
- Landscaper rates - paving, retaining walls and labour prices
- Home extension cost calculator - decks, pergolas and extensions
- Renovation quote comparator - paste your concreting quotes for red-flag analysis
Related reading: Carport Cost Australia (2026) (what a carport costs, and when it needs approval); Polished Concrete Floors in Brisbane: Cost & Guide (2026) (what polished concrete floors cost).
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