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Concrete calculator

Cubic metres and 20 kg bags for a slab, a strip footing or a row of post holes, with an allowance for waste. Free, no signup, and one tap turns it into a quote.

What are you pouring?

m

m

mm

%

The answer

Concrete to order 2.64 m³
Or in 20 kg bags 313 bags
Ready-mix, rounded up 2.7 m³
Exact volume, before waste 2.400 m³
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It only fills in the measurements. The working out is the calculator's, and prices are always yours.

Four pours, worked out

Volume is length times width times depth, in metres. The order adds the waste allowance and rounds ready-mix up to the next tenth of a cubic metre. Bags are the order divided by 0.0084 m³, the yield of a 20 kg bag of Cement Australia Concrete Mix from its product sheet (read 9 October 2026), rounded up.

A garden shed slab

3 m by 3 m, 100 mm thick
What are you pouring? Slab
Length 3 m
Width 3 m
Thickness 100 mm
Extra for waste and uneven ground 10 %
Or in 20 kg bags 118 bags
Ready-mix, rounded up 1.0 m³
Exact volume, before waste 0.900 m³
Concrete to order 0.99 m³

Just under a cubic metre, which is the point where a short-load truck starts to beat 118 bags and a borrowed mixer.

A single driveway

12 m by 3.5 m, 125 mm thick
What are you pouring? Slab
Length 12 m
Width 3.5 m
Thickness 125 mm
Extra for waste and uneven ground 10 %
Or in 20 kg bags 685 bags
Ready-mix, rounded up 5.8 m³
Exact volume, before waste 5.250 m³
Concrete to order 5.78 m³

Ready-mix territory. Confirm the thickness and reinforcement with your council's crossover rules before pricing it.

Fence posts

Twelve holes, 250 mm across and 600 mm deep
What are you pouring? Post holes
Number of holes 12
Hole diameter 250 mm
Hole depth 600 mm
Extra for waste and uneven ground 10 %
Or in 20 kg bags 47 bags
Ready-mix, rounded up 0.4 m³
Exact volume, before waste 0.353 m³
Concrete to order 0.39 m³

Small enough for bags. Rapid-set post mixes yield differently, so check the bag if you use one.

A strip footing

18 m of footing, 450 mm wide and 500 mm deep
What are you pouring? Strip footing
Total length of footing 18 m
Width 450 mm
Depth 500 mm
Extra for waste and uneven ground 10 %
Or in 20 kg bags 528 bags
Ready-mix, rounded up 4.5 m³
Exact volume, before waste 4.050 m³
Concrete to order 4.46 m³

The engineer sets footing sizes. This only turns their drawing into an order.

Concrete questions#

How do I work out how much concrete I need?
Multiply length by width by depth, all in metres. A slab 6 m by 4 m and 100 mm thick is 6 x 4 x 0.1 = 2.4 cubic metres. Then add 5 to 10% for uneven ground, spillage and the formwork bowing, which is what the waste box above does.
How many 20 kg bags of concrete are in a cubic metre?
About 119 by Cement Australia's figure for its Concrete Mix: two 20 kg bags fill a 450 by 500 mm slab 75 mm thick. Other brands quote closer to 100, so check the bag you are buying. This calculator uses the Cement Australia figure, which errs towards having enough.
When should I order ready-mix instead of bags?
Once the job is more than about half a cubic metre. That is around 60 bags to carry and mix, and a truck is usually cheaper and gives a more even pour. Below that, bags are easier. Ask the supplier about their minimum load and any short-load fee.
How thick should a slab be?
Paths and garden shed slabs are commonly 75 to 100 mm, driveways 100 to 125 mm, and house slabs are set by the engineer. Anything structural is the engineer's call, not a calculator's.
How much concrete for a fence post?
Choose Post holes, enter how many, the hole diameter and the depth. A 250 mm hole 600 mm deep takes about 0.03 cubic metres, roughly four 20 kg bags before waste.