How do I calculate concrete for a slab?

Multiply length by width by thickness, with all three in metres. A 5 by 4 metre slab at 100 millimetres is 5 x 4 x 0.1, which is 2 cubic metres. Then add a waste allowance, usually around 10 percent.

Why add a waste allowance?

Because ground is never perfectly level, formwork flexes, and some concrete is lost in the barrow or the pump line. Running short mid-pour creates a cold joint, which is a permanent weakness in the slab, so a little over is much cheaper than a little under.

How many bags of concrete are in a cubic metre?

Roughly 105 bags of 20 kilogram premix, since each yields about 0.0095 cubic metres. Check the figure on the bag, because it varies by product. Above about half a cubic metre, ready-mix delivered is usually both cheaper and better.

Concrete Slab Calculator NZ

This works out how much concrete a rectangular slab needs. The volume itself is simple, length times width times thickness, and the reason a calculator helps is everything around it. Thickness is quoted in millimetres while the slab is measured in metres, so the unit conversion is where the mistakes happen, and being out by a factor of ten on a pour is expensive in a way that is difficult to fix afterwards. The calculator also adds a waste allowance, which is not optional in practice. Ground is never perfectly level, formwork flexes, some is lost in the barrow and in the pump line, and running short mid-pour is far worse than having a little over, because a cold joint in the middle of a slab is a permanent weakness rather than an inconvenience. Ten percent is the usual allowance and it is adjustable here. You get the volume before and after waste, the weight in tonnes, which matters for access and for whether a truck can reach the site, and the equivalent in premix bags for a small job. For anything beyond a few cubic metres, ready-mix delivered is normally both cheaper and better, and the bag figure is mostly there to show you where that line falls.

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Length (m)

Width (m)

Thickness (mm)

Waste allowance

Yield per premix bag (m3)

2.2 m3

concrete needed including waste

Volume before waste2 m3

Weight5.28 t

Premix bags (20 kg)232

Slab area20 m2

Order to nearest 0.1 m32.2 m3

An estimate for planning. Assumes a level base and uniform thickness, neither of which is quite true on site. Concrete is taken at 2,400 kg per cubic metre; confirm the bag yield on the product you are using.

How it works

Volume is the length multiplied by the width multiplied by the thickness, with the thickness converted from millimetres to metres by dividing by 1,000. The waste allowance is added as a percentage on top. Weight uses a density of 2,400 kilograms per cubic metre, which is typical for normal-weight concrete, and is applied to the volume including waste. The bag count divides the volume including waste by the yield of a single bag, which for a 20 kilogram premix bag is usually somewhere near 0.0095 cubic metres and is worth checking on the bag itself, since it varies by product.

Worked example

Take the defaults: a slab 5 metres by 4 metres at 100 millimetres thick. That is 5 x 4 x 0.1, which is 2 cubic metres, over an area of 20 square metres. Adding 10 percent for waste gives 2.2 cubic metres, which is what you would order. At 2,400 kilograms per cubic metre that is 5.28 tonnes. In 20 kilogram premix bags at 0.0095 cubic metres each it would take about 232 bags, which is roughly four and a half tonnes of bags to carry and mix by hand. That comparison is why ready-mix is normally the answer above about half a cubic metre.

Related calculators

Concrete: other shapes including footings and posts.
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Kilograms to Pounds: converting the weight figures.
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Data sources: concrete density is taken at 2,400 kg per cubic metre, typical for normal-weight concrete. Premix bag yield varies by product and should be confirmed on the bag.