How the numbers are worked out
The volume is length × width × thickness, with the thickness turned from millimetres into metres. The extra goes on top, then the order is turned into bags at the maker’s yield and into ready-mix in the supplier’s steps.
Worked example: a 6 × 3 m driveway, 100 mm thick
- Volume: 6 × 3 × 0.1 = 1.8 m³.
- With 10% extra: 1.8 × 1.1 = 1.98 m³.
- Bags: 1.98 × 100 = 198 bags; at 108 a cubic metre, 213.84, so 214; at 110, 217.8, so 218. At $9.90 a bag, 198 bags cost $1,960.
- Ready-mix: 2.0 m³. That’s 1.0 m³ under Boral’s 3 m³ minimum in metropolitan NSW, so about $95 plus GST in short-load fees there.
- Weight in place: 1.8 × 2,400 = 4,320 kg, about 4.3 tonnes.
- Mesh: stopped 40 mm short of each edge, it covers 5.92 × 2.92 m: two 6 × 2.4 m sheets side by side, lapped 225 mm. Two sheets of SL72 weigh 82 kg, held up by 45 bar chairs at 800 mm.
- Joints: one across the middle, for two 3 × 3 m panels.
The extra covers spillage and uneven ground. Hunter Readymixed suggests 10% for wastage on small jobs; change it if your subgrade is flat and your formwork true.
How thick, and how strong
CCAA’s data sheet on residential driveways and paths sets the minimum thickness by the traffic:
- 75 mm for foot traffic only: paths and patios.
- 100 mm for vehicles under 3 tonnes: the family car.
- 150 mm for vehicles of 3 to 10 tonnes, with heavier ones only now and then.
Most residential paving is N20, normal-class concrete of at least 20 MPa. CCAA recommends N25 where vehicles of 3 to 10 tonnes use the pavement, and notes that AS 3600 calls for N32 under pneumatic-tyred vehicles over 3 tonnes. Bagged mixes state their own strengths: Easy Mix Concrete over 25 MPa, Australian Builders High Strength over 40 MPa at 28 days, Dingo MaxPRO 50 MPa.
Under the slab, CCAA recommends a 100 mm sub-base in most cases, with a 20 mm layer of sand over the whole area; Bunnings’ guide to a small slab uses at least 50 mm of compacted crushed rock. CCAA asks for residential paving to be cured for at least 3 days.
Bags or ready-mix
A 20 kg bag makes a little over 0.009 m³ to 0.01 m³: makers state 100 (Australian Builders, Easy Mix), 108 (Boral, Dingo MaxPRO) or about 110 (Cement Australia’s PROSTRENGTH Rapid Set) bags a cubic metre. Choose your bag’s figure in the calculator; the price follows it where Bunnings lists a bag with that yield ($9.90 at 100, $14.44 at 108).
Ready-mix is ordered by the cubic metre in steps of 0.1 m³, rounded up, as Hunter Readymixed takes orders. Enter the price your supplier quotes to compare it with the bags, and remember the short-load fee: under 3 m³, Boral’s metropolitan NSW fees add $95 plus GST for each cubic metre short. The calculator also flags any job of 50 bags or more, a tonne of concrete to lift and mix by hand.
Mesh, bar chairs and joints
CCAA’s rules for the steel in residential paving:
- Mesh in the top half of the slab, with at least 30 mm of concrete over it.
- 40 mm of cover at slab edges and joints.
- Sheets lapped by at least two cross wires: the two outermost wires of each sheet overlap.
- Bar chairs no more than 800 mm apart, closer for light mesh such as SL62.
The slab calculator assumes 6.0 × 2.4 m sheets, lapped 225 mm (RMIT’s BuildRight minimum for square mesh) and stopped 40 mm inside the formwork, as in Bunnings’ guide. The mesh calculator draws the layout and takes other sheet sizes and laps.
Control joints control where the slab cracks. CCAA puts them no more than 3 m apart and keeps every panel within 1.5 times as long as it is wide; the calculator finds the fewest panels that meet both rules and gives their size.
House slabs are designed, not estimated
The slab and footings of a house follow the NCC’s housing provisions, which set N20 concrete with 20 mm aggregate and 100 mm slump and cover such as 40 mm to unprotected ground, or an engineer’s design. Use this calculator for the quantities, and the plans for everything else.
Questions people ask
How many bags of concrete for 1 m² at 100 mm thick?
A square metre 100 mm thick holds 0.1 m³ of concrete: 10 bags at 100 bags a cubic metre, or 11 at 108 or 110 (10.8 and 11, rounded up). With 10% extra it’s 11, 12 or 13 bags.
How much concrete do I need for a 6 × 3 m slab?
At 100 mm thick, 1.8 m³; with 10% extra, 1.98 m³. That’s 2.0 m³ of ready-mix, or 198 to 218 bags of 20 kg, depending on the bag’s yield. At 75 mm it’s 1.35 m³ before the extra; at 150 mm, 2.7 m³.
Is 100 mm thick enough for a driveway?
For cars, yes: CCAA’s minimum for vehicles under 3 tonnes is 100 mm. Vehicles of 3 to 10 tonnes need at least 150 mm, and paths and patios with foot traffic only can be 75 mm.
Do I need mesh in a concrete path?
Mesh doesn’t stop concrete cracking: CCAA describes its job as holding tight any cracks that form. Its data sheet asks for mesh sized to the slab’s thickness and joint spacing, and Bunnings’ guide to a small slab lays mesh too. If you leave it out, the joints matter even more.
How far apart should control joints be?
No more than 3 m apart, says CCAA, and placed so no panel is more than 1.5 times as long as it is wide. A 10 m path 1.2 m wide therefore needs five joints, not three: 2.5 m panels would be more than twice as long as they are wide.
What is a short-load fee?
A charge for delivering less than a full minimum load. Boral’s metropolitan NSW fees charge a load under 3 m³ as if it were 3 m³, at $95 for each cubic metre short, plus GST: on 2.0 m³ that’s $95. Other suppliers set their own minimums and rates.
Sources for the numbers on this page
- Cement Concrete & Aggregates Australia — Residential concrete driveways and paths, data sheet (August 2026, PDF) · checked 4 October 2026
- Building Products Supplies (Australian Builders) — High Strength Concrete 20 kg, product data sheet: about 100 bags make a cubic metre (PDF) · checked 4 October 2026
- River Sands (Easy Mix) — Easy Mix Concrete, data sheet (March 2020): 0.01 m³ a 20 kg bag · checked 4 October 2026
- Boral — Concrete Mix 20 kg, product data sheet (February 2022): 108 bags make a cubic metre · checked 4 October 2026
- Bunnings — Dingo 20kg MaxPRO Concrete Mix: 108 bags make a cubic metre ($14.44) · checked 4 October 2026
- Cement Australia — PROSTRENGTH Rapid Set Concrete, data sheet (April 2024): about 110 bags a cubic metre (PDF) · checked 4 October 2026
- Bunnings — Australian Builders 20kg High Strength Concrete Mix ($9.90) · checked 4 October 2026
- Bunnings — Bastion 20kg Concrete Mix ($8.88) · checked 4 October 2026
- Hunter Readymixed Concrete — Ordering concrete (in 0.1 m³ steps, with 10% for wastage on small jobs) · checked 4 October 2026
- Boral — Concrete service fees, NSW Metro (effective 4 May 2026): loads under 3 m³ · checked 4 October 2026
- Holcim Australia — Glossary: density of concrete and normal-class grades (N20 to N50) · checked 4 October 2026
- Australian Building Codes Board — NCC 2022 Housing Provisions, Part 4.2: footings, slabs and associated elements (laps, bar chairs, cover, N20) · checked 4 October 2026
- Bunnings — How to pour a small concrete slab (mesh 40 mm inside the formwork, 40 mm over the mesh) · checked 4 October 2026
- InfraBuild — Ribbed square mesh: SL62 to SL102 wire sizes and mass per sheet · checked 4 October 2026
- RMIT University, BuildRight — Concrete slab reinforcement: lapping square mesh (at least 225 mm) · checked 4 October 2026
Corrections and updates are logged on the sources page.