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Concrete calculator

A 3x3m shed slab 100mm thick needs 0.95 m³ of concrete including 5% extra, which is about 104 x 20kg bags: a job you can do from bags or with a small truck.

In US sizes a 12x10ft slab at 4in is 1.56 yd³ (42 ft³), about 73 x 80lb bags.

1. What are you doing?

2. Shape

Shape

Then tap a size on the picture to type it.

3. Your job

Tap to type this size3mTap to type this size3mTap to type this size100mm

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

4. Sizes

Product

Allmat concrete calculator

104 x 20kg Concrete Mix 20kg If you order ready-mix, ask for about 1 m³.

What we'd use

  • Concrete Mix 20kg bag

    Concrete Mix 20kg: 104 bags

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.

  • 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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The frame sizes itself to the calculator; the small script handles that. The link under the frame can stay or go.

How much concrete do I need for the whole job?

Most jobs have more than one part. Add each one to the list and the calculator totals them by product. For example, 6 fence posts plus a 4x1m path 75mm thick come to 0.52 m³ with the extra allowed, split into 24 bags of Rapid Set and 35 bags of Concrete Mix.

One link holds the whole list, so you can send it to whoever is helping with the job and they see exactly what you see.

Concrete volume formulas for every shape

Every answer starts with volume. These are the sums the calculator does for you, in metres:

  • Slab, path or footing: length × width × depth.
  • Sloped slabs, post holes, round piers and solid steps each add a twist; their own pages show the working step by step.

Concrete bag calculator: any bag size

Cement Australia's figure is about 110 x 20kg bags to make 1 m³. For other bags the calculator estimates from weight, so a 25kg bag makes about 11.4 litres and an 80lb bag about 16.5 litres. If your bag prints its own yield, use that figure to check the count.

When bags stop making sense

Up to about a cubic metre, plenty of people still mix from bags; past that, and for a house slab or any driveway beyond a small patch, order a ready-mix truck. The ready-mix page shows how much to order and how suppliers round it.

Questions

Measure the length, width and depth in metres and multiply them to get cubic metres, then add 5 to 10% for spillage and uneven ground. The calculator does this for every shape and adds the parts of a job together.

Yes. Switch to Imperial (US) and every size, volume and bag count changes to feet, inches, cubic yards and pound bags.

Yes. Use "Add another part to this job" for each piece. The answer totals the concrete by product and one link saves the whole list.

Order them through Allmat from the result: we pick and pack the bags, then it is 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

Last checked 5 October 2026.

Cite this page

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

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