Concrete slab calculator

A 3x3m slab at 100mm takes 0.95 m³ with 5% extra, or 104 x 20kg bags of Concrete Mix. A 12x10ft patio at 4in comes to 1.56 yd³ (42 ft³).

1. What are you doing?

2. Shape

Then tap a size on the picture to type it.

3. Your job

Tap a size to type it. Use Plan to drag the corners or draw your own shape.

4. Sizes

Length (m)

Width (m)

Depth (mm)

Sloped (deeper at one end)

Product

Extra allowed (%)

Allmat concrete calculator

What we'd use

Concrete Mix 20kg: 104 bags

Allow an equivalent brand to specification

You need 0.95 m³ of concrete

0.9 m³ plus extra for spillage and uneven ground (5% on the slab or pad).

Or order a ready-mix truck: ask for about 1 m³.

Details: other bag sizes, water, weight, getting it home

About 104 x 20kg or 84 x 25kg bags of concrete, our estimate from bag weight.

Concrete Mix 20kg (104 × 20 kg): Water: about 182 L. Weight: 2,080kg. Mixer loads at 2 bags a load: 52.

That adds 10% and rounds up to the next step a supplier sells.

Cement Australia's guide: under 0.1 m³ use bags, 0.1 to 0.5 m³ mix your own, over 0.5 m³ ready-mix. Up to about 1 m³ we still show the bags; past that, and for a house slab or a driveway beyond a small patch, it is a truck pour.

Getting it home

Total weight: 2,080kg. About 3 ute loads at 1,000kg each, our estimate.

The load alone is over 750kg, the limit for an unbraked trailer, and the trailer's own weight counts too.

Click and collect or delivered across Adelaide.

How we worked it out

Slab or pad: 0.9 m³ plus 5% = 0.95 m³, ordered as ready-mix.

Cement Australia: about 110 x 20kg bags make 1 m³ (their FAQ, calculator and the PROSTRENGTH Rapid Set data sheet). Rapid Set Concrete publishes no per-m³ figure, so it uses the same 110.

Trade Mortar: about 20 house bricks or 10 masonry blocks per 20kg bag at a 10mm joint (Cement Australia).

Estimates only. Check sizes and quantities against your plans, and with your engineer or builder, before you order.

Pour-day check

Type in the weather for the day you plan to pour. Temperature never changes how much you need; it changes the timing.

Air temp (°C)

Wind (km/h)

Humidity (%)

Concrete Mix 20kg: 20°C is within the maker's range (5°C to 35°C).
Ready-mix (house slab or driveway): AS 1379 requires the concrete to arrive between 5°C and 35°C; outside that, talk to the plant (CCAA).
Keep it damp for 7 days (Cement Australia).

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How thick should a concrete slab be?

Cement Australia gives 75mm as the minimum for a slab that carries no load, such as a garden path or a small shed pad. Driveways and anything a car sits on need more, and the calculator starts a driveway at 100mm.

A slab that holds up part of a building is designed by an engineer; use this calculator for the quantity once you have the drawing. For a house floor with edge beams or waffle pods, use the house slab calculator.

Concrete for common slab sizes

Each figure includes the 5% extra the calculator adds to slabs. Past about 1 m³ most people order a ready-mix truck instead of bags, and the calculator says so.

Concrete for common slab sizes, including 5% extra, with 20kg bags of Concrete Mix
Slab	75mm	100mm	125mm
2x1m	0.16 m³, 18x20kg	0.21 m³, 24x20kg	0.26 m³, 29x20kg
3x3m	0.71 m³, 78x20kg	0.95 m³, 104x20kg	1.18 m³, 130x20kg
4x3m	0.95 m³, 104x20kg	1.26 m³, 139x20kg	1.58 m³, 174x20kg
5x4m	1.58 m³, 174x20kg	2.1 m³, 231x20kg	2.63 m³, 289x20kg
6x3m	1.42 m³, 156x20kg	1.89 m³, 208x20kg	2.36 m³, 260x20kg

Concrete slab sizes in feet and inches

The same sums for slabs measured in feet, counted in 80lb bags.

Concrete for common slab sizes, including 5% extra, with 80lb bags (our estimate from bag weight)
Slab	3.5in	4in	5in
6x3ft	0.2 yd³, 10x80lb	0.23 yd³, 11x80lb	0.29 yd³, 14x80lb
10x10ft	1.13 yd³, 53x80lb	1.3 yd³, 61x80lb	1.62 yd³, 76x80lb
12x10ft	1.36 yd³, 64x80lb	1.56 yd³, 73x80lb	1.94 yd³, 91x80lb
16x12ft	2.18 yd³, 101x80lb	2.49 yd³, 116x80lb	3.11 yd³, 145x80lb
20x10ft	2.27 yd³, 106x80lb	2.59 yd³, 121x80lb	3.24 yd³, 151x80lb

Sloped and odd-shaped slabs

A slab that falls from one end to the other uses the average depth. A 3x3m slab running from 100mm to 150mm works out at 1.18 m³ including the extra.

For an L, U or T shape, choose "Draw a shape", drag the corners or type each edge, and the area updates as you go.

Before you pour a slab

Cement Australia's slab guide sets out the groundwork: compacted road base under the slab, formwork as deep as the slab, mesh sitting about 50mm above the base, and control joints cut about every 1.5m.

Questions

At 100mm thick it is 0.9 m³ before any extra, about 99 x 20kg bags of Concrete Mix, or 104 bags with the 5% the calculator adds.

Cement Australia gives 75mm as the minimum for a slab with no load on it, like a path. Use more for anything a vehicle drives on.

Multiply the length by the width by the average of the two end depths. Tick "Sloped" on a rectangle and enter both depths.

A 6x3m driveway at 100mm needs about 1.89 m³ with extra, which would be 208 x 20kg bags. A driveway is poured in one go, so order ready-mix: the calculator gives the truck figure rather than bags.

Yes. Order through Allmat from the result, and the bags are click and collect or delivered across Adelaide.

Goes with this job

Disclaimer

This calculator gives estimates from the sizes you enter, using manufacturers' published figures and standard geometry. It is not engineering or design advice. Footing, slab and beam sizes must come from your plans or engineer. Allow for site conditions, spillage and uneven ground. Check quantities with your supplier before ordering. Allmat accepts no responsibility for decisions made using these estimates.

Sources

Cement Australia: slab calculator
Cement Australia: how to lay a concrete slab
Cement Australia: Concrete Mix
Allmat arithmetic (this calculator)

Last checked 5 October 2026.

Cite this page

Allmat, "Concrete slab calculator", https://allmat.com.au/concrete-calculator/slab, checked 5 October 2026.