How do I calculate concrete volume?

Multiply length by width by thickness, all in metres. For a slab measured with thickness in millimetres, divide the thickness by 1,000 first. For example, a 4 m by 3 m slab at 100 mm thick is 4 x 3 x 0.1, which is 1.2 cubic metres.

How many bags of concrete are in a cubic metre?

As a rough guide, a 20kg bag of premix concrete yields about 0.009 cubic metres, so you need roughly 110 bags per cubic metre. A 25kg bag yields about 0.0112 cubic metres, roughly 88 bags per cubic metre. Always check the yield printed on the bag, as it varies by product.

When should I order ready-mix instead of bags?

Bags are practical for small jobs up to roughly half a cubic metre. Beyond about 0.5 cubic metres, ordering ready-mix concrete delivered by truck is usually cheaper, faster, and gives a more consistent mix. Most suppliers have a minimum order.

Why add a wastage allowance?

Some concrete is always lost to spillage, uneven ground, and over-filling. A wastage allowance of about 5 to 10% means you are not caught short part way through a pour, which can ruin the finish.

Concrete Calculator

Work out how much concrete you need for a slab, path, or footing. Enter the length and width in metres and the thickness in millimetres, and the calculator gives the volume in cubic metres plus a wastage allowance. It also estimates how many 20kg or 25kg premix bags that volume needs, so you know whether to buy bags or order ready-mix.

Length

Width

Thickness

Wastage

1.32 m³

concrete needed (including wastage)

20kg bags147

25kg bags118

Base volume1.2 m³

Bag counts use typical yields (20kg bag about 0.009 m³, 25kg bag about 0.0112 m³). Check the yield on your chosen product. For pours over about 0.5 m³, ready-mix is usually better value.

How to calculate concrete volume

Concrete volume is simply length times width times thickness, with every measurement in the same unit. Because slab thickness is usually given in millimetres, divide it by 1,000 to convert to metres first. The result is the volume in cubic metres, which is how ready-mix concrete is ordered.

Worked New Zealand example

You are pouring a 4 m by 3 m shed slab at 100 mm thick. The base volume is 4 x 3 x 0.1, which is 1.2 cubic metres. Adding a 10% wastage allowance brings it to about 1.32 cubic metres.

That is well over half a cubic metre, so ready-mix delivered by truck would be the sensible choice rather than mixing roughly 147 bags by hand.

Bags or ready-mix?

Bags suit small jobs: post holes, small paths, repairs, up to about 0.5 cubic metres.
Ready-mix suits larger pours: driveways, slabs, and floors. It is cheaper per cubic metre at volume and gives a consistent mix.
Always add a wastage allowance of 5 to 10% so you do not run short mid-pour.

Frequently asked questions

How many 20kg bags are in a cubic metre?

Roughly 110, because a 20kg bag yields about 0.009 cubic metres. A 25kg bag yields about 0.0112 cubic metres, so about 88 per cubic metre. Confirm the figure on the bag.

What thickness should a slab be?

That depends on the use and must meet the relevant building requirements. A common domestic path or shed slab is 100 mm, but always follow the engineering or council specification for your project.

Related calculators

Cubic Metre Calculator: volume of any box-shaped space.
Square Metre Calculator: floor area in square metres.
Volume Conversion Calculator: convert between volume units.
Area Conversion Calculator: convert between area units.

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