Concrete Slab Calculator
How much concrete a slab needs from its length, width and depth, with waste added, as cubic metres and 20 kg bags — and what it costs as ready-mix or in bags.
Volume = length × width × depth (in metres), plus any extra for footings or thickened edges.
| Slab volume | 2.4 |
|---|---|
| Waste | 0.24 |
How the concrete slab calculator works
Volume = length × width × depth (in metres), plus any extra for footings or thickened edges. Add the waste allowance for spillage and uneven ground to get the amount to order. A 20 kg bag yields about 0.01 m³, so bags = order ÷ 0.01, rounded up.
Formula: Base m³ = L × W × depth ÷ 1000 + extra · order = base × (1 + waste%) · bags = ⌈order ÷ 0.01⌉
How the concrete volume is worked out
Volume = length × width × depth, then add waste
The calculator takes your slab's plan area (length × width) and multiplies it by the depth in metres to get the raw volume, then adds any extra volume for footings or post holes. It then adds a waste allowance — the concrete you actually order is volume × (1 + waste). A 20 kg bag of pre-mixed concrete yields only about 0.01 m³, so a cubic metre is roughly 100 bags — nearer 110 once you include waste.
Worked example
A 20 m² floor poured 100 mm deep is 20 × 0.1 = 2.0 m³ before waste. Add the standard 10% and you order about 2.2 m³ — that's roughly 220 × 20 kg bags, or about $770 as ready-mix at $350/m³. Depth drives the number directly: the same slab at 150 mm would be 50% more concrete.
Slab depthReady-mix vs bags, minimum loads and reinforcement
Bags vs a concrete truck
Bagged concrete works out to roughly $850–$1,300 per cubic metre once you count the bags, and it's a lot of hand-mixing. Ready-mix is about $320–$490 per cubic metre delivered (an N25 mix is often around $340–$370). Bags only make sense for small jobs — post holes, a small landing — where a truck's fees don't stack up. Past about a cubic metre, order ready-mix.
Minimum loads & short-load fees
Concrete trucks carry up to about 8 m³ but charge a minimum load (often around 1 m³) and a short-load surcharge under roughly 2–3 m³, plus delivery and any waiting time — so a small pour costs more per cubic metre than a big one. Ask for the price delivered, and batch pours where you can.
What this tool does not size
This calculator estimates volume and a rough cost only. You still need to specify the mix strength (MPa) and slump, and any reinforcing mesh — for example SL72 — with your engineer and batching plant. Reinforcement and mix spec don't change the volume, but they're essential to a slab that lasts.
Frequently asked Frequently asked questions
Where these figures come from
This is a geometry-based estimate — the concrete volume comes from your slab's dimensions, not from any external rate table. The guidance below reflects Australian standards and typical trade practice.
- Residential slab design — slabs and footings for houses in Australia are designed to AS 2870 (Residential slabs and footings), which sets the classification of reactive soils and the slab type. Construction is regulated under the National Construction Code (NCC).
- Bag yield and quantities — a 20 kg bag of concrete mix yields about 0.01 m³, so roughly 100 bags per cubic metre; confirm the yield printed on the bag you buy.
- Prices — ready-mix around $320–$490/m³ delivered and bagged concrete around $850–$1,300/m³ are approximate market ranges that vary with region, mix strength, load size and delivery; they are not a quote.
- Further guidance — ACCC — consumer rights on quotes, contracts and guarantees.
Last checked: July 2026. Prices are indicative and vary by region, mix and site. Always confirm the mix strength, reinforcement and a delivered price with your engineer and batching plant.
Understanding your resultSelect the question that matches where you are right now.
The headline number is the concrete to order: the slab's volume plus a waste allowance for spillage and an uneven subgrade. The breakdown also converts it to 20 kg bags and, if you enter prices, to a ready-mix and a bagged cost.
What to do with itOrder this volume from the batching plant (or buy the bags). Give them the mix strength and slump your engineer specifies — the volume is what you order, the mix is a separate line.
What it is notIt's not a structural design or a full quote. It doesn't size reinforcement, footings depth, or the mix strength, and it doesn't include pump hire, formwork or labour.
AccuracyThe volume maths (area × depth) is exact; real pours vary with an uneven subgrade and spillage, which is what the waste allowance absorbs. Confirm the bag yield and delivered price with your supplier.
Four things move the volume: the area, the depth, the waste allowance, and any extra volume for footings.
Depth dominatesVolume scales straight with depth: a slab at 150 mm is 50% more concrete than the same slab at 100 mm. Getting the depth right — from AS 2870 for a structural slab — matters more than any other input.
Waste allowance5–10% covers spillage and an over-dug or uneven subgrade. Lean towards 10% on rough ground — a subgrade 20 mm deeper than planned across a big slab is real extra concrete.
Footings, thickenings and post holesSlabs are rarely a flat rectangle. Edge beams, thickenings under walls and separate post holes all add concrete — a 300 mm-diameter post hole 600 mm deep is about 0.042 m³. Add these under the extra-volume field so the order isn't short.
A few habits stop you over- or under-ordering.
Measure the dig, not the planMeasure the actual formed-up area and check the subgrade depth at several points — over-dig adds concrete. Use the real dimensions and let the waste allowance cover the rest.
Round the order upRunning short mid-pour means a cold joint or a second short-load delivery fee. A little concrete left over is always cheaper than stopping the pour.
Spec the mix and steelVolume is only half the order. Tell the plant the MPa (N20–N32), slump and any reinforcement (e.g. SL72 mesh) your engineer specifies before the truck rolls.
A slab is usually one line in a bigger renovation budget. Model the surface and the money side too.
Tiling the slabWorking out floor or wall tiles for the finished area?
Tile calculator → How much can you borrow?Check your borrowing capacity before you commit to a bigger job.
Borrowing capacity → Financing the renoSee what monthly repayments a renovation adds to your home loan.
Mortgage repayments → Related calculators 🧩Tile CalculatorTiles, boxes & cost 🧾Cost of LivingHousehold running costs 🏠Mortgage RepaymentFinance a reno 🔑First Home BuyerBuying to renovate 📊Borrowing CapacityHow much you can borrow ⛽Fuel CostTrips to the yard You might also be interested in 🧩Tile CalculatorFinish the floor 🧾Cost of LivingBudget the whole home 🏠Mortgage RepaymentWhat a reno loan costs 📊Borrowing CapacityBefore you commit 🔑First Home BuyerBuying a doer-upper ⛽Fuel CostTrips to the yard Rate this calculatorQuick answers
A 4×5 m slab at 100 mm About 2.2 m³ of concrete
A 20 m² slab poured 100 mm deep is 2.0 m³ of concrete before waste — add the usual 10% for spillage and uneven ground and you order about 2.2 m³. As ready-mix at roughly $350/m³ that's near $770 delivered; as 20 kg bags it's about 220 bags, which is far dearer and a lot of mixing.
| Area × depth | 20 m² × 100 mm |
|---|---|
| Base volume | 2.0 m³ |
| With 10% waste | ≈2.2 m³ |
| Ready-mix ≈ | $770 (at $350/m³) |
- In bags ≈220 × 20 kg (≈$1,980)
- Lesson Ready-mix wins past ~1 m³
A 30 m² driveway at 100 mm About 3.3 m³
Driveways and shed floors are typically 100 mm. A 30 m² driveway is 3.0 m³ before waste, about 3.3 m³ ordered — around $1,155 as ready-mix. At this size a concrete truck is the only sensible option; that's 330 bags otherwise. Driveways bearing vehicles usually want reinforcing mesh (e.g. SL72) and control joints too.
| Area | 30 m² |
|---|---|
| Depth | 100 mm |
| Order volume | ≈3.3 m³ |
| Ready-mix ≈ | $1,155 |
- Reinforcement Mesh (SL72) + control joints
- Bags? ≈330 — not practical
How deep should the slab be? 75 mm paths · 100 mm slabs · 125 mm+ clay
Depth drives the volume directly. Common Australian residential thicknesses: garden paths and footpaths 75 mm, house slabs, driveways and shed floors 100 mm, and 125–150 mm where soils are reactive clay or the slab carries heavy loads. Always follow your engineer's or the NCC/AS 2870 requirement for the actual site — depth is not a guess on a structural slab.
| Paths | 75 mm |
|---|---|
| Slabs / driveways | 100 mm |
| Reactive clay / heavy | 125–150 mm |
- Volume scales 150 mm = 50% more than 100 mm
- Structural slabs Follow the engineer / AS 2870
Bags or a concrete truck? Ready-mix wins past about 1 m³
A 20 kg bag yields only about 0.01 m³, so a cubic metre is around 100 bags — and bagged concrete costs roughly $850–$1,300/m³ versus ready-mix at about $320–$490/m³. Bags suit small jobs (post holes, a tiny landing) where a truck's minimum load and short-load fees don't stack up. Anything over about a cubic metre, order ready-mix.
| 20 kg bag yields | ≈0.01 m³ |
|---|---|
| Bags per m³ | ≈100 (110 with waste) |
| Bagged $/m³ | $850–$1,300 |
| Ready-mix $/m³ | $320–$490 |
- Bags for Post holes, tiny pours
- Ready-mix for Slabs, driveways, paths
Ready-mix minimum load & short-load fees Trucks charge for small pours
Concrete trucks carry up to about 8 m³ but charge a minimum load (often around 1 m³) and a 'short-load' surcharge under roughly 2–3 m³, plus delivery and any waiting time. That's why a 1.5 m³ pour can cost more per cubic metre than a 5 m³ one. Batch your pours where you can, and get the price 'delivered', not just the per-m³ rate.
| Truck capacity | ≈8 m³ |
|---|---|
| Minimum load | often ≈1 m³ |
| Short-load fee | under ~2–3 m³ |
- Ask for The delivered price, all-in
- Pump hire Extra if access is tight
How much waste to allow 5–10% on most slabs
Add 5–10% to the theoretical volume for spillage, an uneven or over-dug subgrade, and slight over-ordering (running short mid-pour means a cold joint or a second delivery fee). Uneven ground is the big one — a subgrade that's 20 mm deeper than planned across a big slab is real extra concrete. Round the order up; a little left over beats stopping the pour.
| Typical allowance | 5–10% |
|---|---|
| Rough / dug-out ground | Lean to 10%+ |
| Why | Uneven subgrade + spillage |
- Running short Cold joint or a 2nd load fee
- Set it At the Standard precision level
Footings, post holes & thickenings Add them as extra volume
Slabs often aren't a flat rectangle — edge beams and thickenings under walls, and separate post holes for a pergola or fence, add concrete. A single 300 mm-diameter post hole 600 mm deep is about 0.042 m³; four of them ≈0.17 m³. Add these under 'extra volume', or calculate the slab and footings separately and sum them.
| Post hole 300⌀×600 | ≈0.042 m³ |
|---|---|
| Edge thickenings | Add as extra volume |
| Where | 'Extra area' field, Detailed level |
- Formula π × r² × depth per hole
- Tip Round each hole up — augers over-cut
What concrete strength (MPa)? N20–N25 for most house slabs
Concrete is specified by strength: N20 (20 MPa) for paths and non-structural work, N25 for most house slabs and driveways, N32 for heavier-duty or exposed work. Higher MPa costs a little more per cubic metre. This calculator estimates VOLUME and rough cost — tell the batching plant the MPa, slump and any additives your engineer or the NCC specifies; the volume is the same, the mix spec is separate.
| Paths | N20 (20 MPa) |
|---|---|
| House slabs / driveways | N25 (25 MPa) |
| Heavy-duty | N32+ |
- Volume Same regardless of MPa
- Specify MPa + slump when ordering
Worked examples
| Inputs | Concrete to order | Note |
|---|---|---|
| 6 m × 4 m at 100 mm, 10% waste | 2.64 m³ | 2.64 m³ to order; 265 bags |
| 10 m × 5 m at 150 mm, 5% waste | 7.88 m³ | 7.875 m³ |
| A custom 120 mm depth with 0.5 m³ of footings | 3.72 m³ | 3.72 m³ |
FAQFrequently asked questions
How deep should the slab be?
Depth is the single biggest lever on volume. Common Australian residential thicknesses are: 75 mm — garden paths and footpaths. 100 mm — house slabs, driveways and shed floors on stable ground. 125–150 mm — reactive clay sites, or slabs carrying heavy loads. These are typical figures, not a specification. A structural slab — anything the house or a heavy load sits on — must be designed to AS 2870 for the actual soil class and built under the NCC . Get the depth, reinforcement and footing details from your engineer; don't size a structural pour off a rule of thumb. Ready-mix vs bags, minimum loads & reinforcement
How much concrete do I need for a slab?
Multiply length × width × depth in metres to get the volume, then add 5–10% for waste. A 4 × 5 m slab at 100 mm is 20 m² × 0.1 m = 2.0 m³ before waste, or about 2.2 m³ once you add the standard 10% for spillage and an uneven subgrade. Round the order up rather than down.
How many 20 kg bags per cubic metre?
A 20 kg bag of concrete mix yields roughly 0.01 m³, so you need about 100 bags per cubic metre — around 110 once you allow for waste. Bags suit small pours; past about a cubic metre, ready-mix is cheaper and far less mixing.
How deep should a concrete slab be?
Common Australian residential thicknesses are 75 mm for garden paths and footpaths, 100 mm for house slabs, driveways and shed floors, and 125–150 mm on reactive clay or under heavy loads. A structural slab's depth must follow your engineer or AS 2870 — it is not a rule-of-thumb guess.
Should I use bags or ready-mix?
Bagged concrete costs roughly $850–$1,300 per cubic metre and is a lot of hand mixing; ready-mix is about $320–$490 per cubic metre delivered. Bags only win for small jobs — post holes, a small landing — where a truck's minimum-load and short-load fees don't stack up. Over about a cubic metre, order ready-mix.
How much extra for waste?
Add 5–10% to the theoretical volume for spillage and an uneven or over-dug subgrade, and round the order up. Running short mid-pour means a cold joint or a second delivery fee, so a little concrete left over is cheaper than stopping the pour.
Does it include reinforcing mesh or the mix strength?
No. This tool sizes the concrete volume and gives a rough cost. You still need to specify the mix strength (for example N20 for paths, N25 for house slabs, N32 for heavy-duty work), the slump, and reinforcement such as SL72 mesh with your engineer or batching plant — none of that changes the volume. Sources
The sources behind this calculator
- American Association of Feline Practitioners — Feline Life Stage Guidelines — the six feline life stages the cat-age page uses
- Wang et al. 2020, Cell Systems — Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome — the 16·ln(age)+31 epigenetic formula on the dog-age page
- Fair Work Ombudsman — sets hospitality award wages — why tipping is optional in Australia
Last checked: July 2026. Where a page states a convention (a tipping custom, a pet life stage), the page names the body it follows.