Concrete Calculator: m³ and 20 kg Bags of Premix

Estimate only. Check product labels and the building rules in your state or territory before you buy or build.

Quick answer

Multiply length × width × thickness in metres to get cubic metres, then add 4 to 10% for waste. A 3 × 3 m slab 100 mm thick is 0.9 m³; with 10% extra that is 0.99 m³, or 118 bags of 20 kg premix at about 8.44 litres per bag. For any job over 0.5 m³, Cement Australia suggests having ready-mix delivered where access allows.

How to use the concrete calculator

Pick what you are pouring: a slab (path, shed slab, pad for a water tank), a footing (a strip or pad under a wall or steps), or post holes and round columns.
Enter the size. Lengths are in metres; thickness, footing width and depth, and hole diameter and depth are in centimetres, so a 100 mm slab is 10 cm.
Read the concrete to order in cubic metres and the number of 20 kg bags of premix. The bag count includes the extra for waste and is rounded up to whole bags.
Open ‘Waste allowance and bag yield’ to change the extra from 10%, or to type the yield printed on the bag you plan to buy.

How do you calculate concrete in m³?

Multiply length by width by thickness, with every measurement in metres. A 3 × 3 m slab 100 mm thick is 3 × 3 × 0.1 = 0.9 m³, which is 900 litres.

For a round hole or column, the cross-section is π × diameter² ÷ 4. Multiply it by the depth and the number of holes, after taking off the part of each hole that a post fills. Then add the waste allowance and divide by the yield of one bag, rounding up to a whole bag.

slab volume = length × width × thicknessfooting volume = length × width × depth × number of footingshole volume = (π × diameter² ÷ 4 − post width²) × depth × number of holesconcrete to order = volume × (1 + waste % ÷ 100)bags = concrete to order ÷ yield of one bag, rounded up1 m³ = 1,000 litres

How many bags of premix for a small slab?

A 2 × 1 m slab 100 mm thick takes 27 bags of 20 kg premix with 10% extra. The table gives the concrete to order for small slabs 100 mm thick, with 10% extra, and the number of 20 kg bags at about 8.44 litres each. The last column applies the rule of thumb in Cement Australia’s slab guide to the concrete to order: bags under 0.1 m³, mixing your own from cement, sand and aggregate between 0.1 and 0.5 m³, and delivered ready-mix over 0.5 m³.

Cubic metres to order and 20 kg bags of premix for a slab 100 mm thick, with 10% extra
Slab (m)	To order	Bags	Guide
0.9 × 0.9	0.089 m³	11	Bags
1.2 × 1.2	0.158 m³	19	Mix your own
2 × 1	0.22 m³	27	Mix your own
2 × 2	0.44 m³	53	Mix your own
3 × 2	0.66 m³	79	Ready-mix
3 × 3	0.99 m³	118	Ready-mix
4 × 3	1.32 m³	157	Ready-mix

For a slab 75 mm thick, multiply the cubic metres by 0.75; for 125 mm, multiply by 1.25. Multiply by 1,000 for litres, then divide by 8.44 and round up for the number of bags.

Worked examples

Premix for a 1.2 × 0.9 m bin pad, 75 mm thick

The slab is 1.2 × 0.9 × 0.075 = 0.081 m³. With 10% extra, plan on 0.089 m³, or 89 litres.

One 20 kg bag makes about 8.44 litres, so 89.1 ÷ 8.44 = 10.6, rounded up to 11 bags, 220 kg in all. That is under the 0.1 m³ that Cement Australia suggests mixing from bags, and at 1.75 litres of water per bag it needs about 19 litres of water.

Concrete for six fence posts

The posts are 90 mm square, in holes 250 mm across with 600 mm of concrete. Cement Australia’s fence post guide adds 10 cm to the depth of each hole for a layer of gravel under the post, which is not part of the concrete.

Each hole has a cross-section of π × 0.25² ÷ 4 = 0.0491 m², less 0.09 × 0.09 = 0.0081 m² for the post, which leaves 0.0410 m². Times 0.6 m and six holes, that is 0.148 m³. With 10% extra it is 0.162 m³: 20 bags of 20 kg premix, about 3.2 bags a hole.

Should I use bags of premix or order ready-mix?

Use bags for less than 0.1 m³ of concrete and have ready-mix delivered for more than 0.5 m³ where access allows, Cement Australia’s slab guide suggests; in between, it suggests mixing your own from cement, sand and aggregate. In bags, 0.1 m³ is 12 bags of 20 kg premix and 0.5 m³ is 60 bags, before any extra for waste.

Ready-mix is sold by the volume of fresh concrete as it is discharged from the truck, so give the supplier the ‘concrete to order’ figure, not the volume of the formwork. The same slab guide says that a job that calls for a specified concrete strength, such as N20/10, needs a professional.

How thick should a concrete slab be?

Cement Australia’s DIY slab guide is written for slabs 100 mm thick, with formwork of 100 × 50 mm timber, a 5 cm bed of compacted crusher dust or road base underneath and control joints every 150 cm. The calculator starts at 100 mm; the crusher dust or road base is not part of the concrete.

Building work in Australia is regulated by the National Construction Code (NCC), which each state and territory adopts and may vary. For a slab under a building, a garage or a driveway, check with your council or a building surveyor or certifier and use a qualified builder or engineer. Their requirements take priority over any figure on this page.

Common questions

How many bags of premix make a cubic metre?

It takes about 119 bags of 20 kg premix to make one cubic metre, at about 8.44 litres per bag. That yield comes from Cement Australia’s coverage figure: 2 bags fill a 450 × 500 mm slab 75 mm thick. With 10% extra for waste, a cubic metre calls for 131 bags, or 2,620 kg of dry mix. Cement Australia suggests ready-mix for any job over 0.5 m³ where access allows.

How many bags of concrete do I need per fence post?

A 90 mm square post in a hole 250 mm across with 600 mm of concrete takes 4 bags of 20 kg premix, with 10% extra. The hole holds 0.025 m³ (25 litres) once the post is taken off, or 27 litres with the extra. Dig 10 cm deeper for a layer of drainage gravel, as Cement Australia’s fence post guide suggests, and leave it out of the depth. A hole 300 mm across holds about half as much concrete again.

How much area does a 20 kg bag of concrete cover?

A 20 kg bag of premix covers about 0.11 m² at 75 mm thick: Cement Australia says 2 bags cover a 450 × 500 mm slab at that thickness. At 100 mm thick a bag covers about 0.084 m², so a square metre of slab 100 mm thick takes 12 bags before waste, or 14 with 10% extra.

How do I calculate concrete for a footing?

Multiply the length of the footing by its width and depth, all in metres. A strip footing 5 m long, 500 mm wide and 300 mm deep is 5 × 0.5 × 0.3 = 0.75 m³, or 0.825 m³ with 10% extra. That is 98 bags of 20 kg premix, well over the 0.5 m³ where Cement Australia suggests ready-mix. For a footing around a building, measure along the centre line so the corners are not counted twice.

How much water does a bag of premix need?

A 20 kg bag of Cement Australia Concrete Mix needs 1.75 litres of clean water, according to the maker’s slab guide. Mix no more than 2 bags at a time in a wheelbarrow, and add more water only if needed, about 100 mL at a time; the maker warns that too much water ruins good concrete. The guide also says a 20 kg bag of cement product takes 2 people to lift safely.

How long before I can walk on a new concrete slab?

You can walk on a new slab after 3 days, but keep heavy loads off it for at least 7 days, Cement Australia’s slab guide says. The formwork can come off after 24 hours. Keep the surface and edges damp with a gentle spray from a hose twice a day for 7 days, which the maker says helps the concrete reach its full strength.

Sources

Cement Australia, Concrete Mix 20kg product data sheet (item 14039) (checked 19 October 2026)2 × 20 kg bags cover a 450 × 500 mm slab approximately 75 mm thick; keep moist for 7 days for maximum strength; formwork off after 24 hours; too much water ruins good concrete.
Cement Australia, How to lay a concrete slab (checked 19 October 2026)Bags under 0.1 m³, mix your own 0.1 to 0.5 m³, ready-mix over 0.5 m³; 1.75 litres of water per bag; 100 × 50 mm formwork for 100 mm slabs; 5 cm of crusher dust or road base; control joints at 150 cm; walk on it after 3 days, no heavy loads for 7 days.
Cement Australia, How to set a fence post (checked 19 October 2026)Dig each hole 10 cm deeper and place 10 cm of gravel under the post for drainage.
Australian Building Codes Board, General NCC FAQ (checked 19 October 2026)The NCC is given legal effect through adoption by each state and territory, which may vary or add to it.
National Ready Mixed Concrete Association (US), CIP 8: Discrepancies in Yield (checked 19 October 2026)Allow 4 to 10% over plan dimensions; ready-mix is sold by the volume of fresh concrete discharged from the truck mixer.

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