How many bags of concrete do you need per cubic metre?
A 20 kg bag of pre-mixed concrete yields roughly 0.0107 cubic metres, so a full cubic metre takes about 93 bags. Knowing that figure is mostly useful for working out when to stop buying bags.
The short answer
About 93 bags of 20 kg pre-mixed concrete per cubic metre, at a yield of roughly 0.0107 m³ per bag. That is close to two tonnes of material to carry and mix by hand, which is why a full cubic metre is well past the point where ready-mix delivery makes more sense.
Work out the volume first
Everything starts with cubic metres, and the arithmetic is length × width × depth with all three in metres.
The conversion is where it goes wrong. A slab 4 metres by 3 metres at 100 millimetres thick is 4 × 3 × 0.1 = 1.2 m³ — the thickness has to become 0.1, not stay as 100. Getting that wrong by a factor of a thousand produces an answer nobody sanity-checks because it is so obviously absurd, which is oddly safer than being wrong by ten per cent.
Post holes are cylinders: π × radius² × depth, less the volume of the post if it is already in the hole when you pour. A 90 mm post in a 300 mm hole 600 mm deep displaces about five litres, which across a fence line adds up to a bag or two.
Bag yield, and why it varies
Around 0.0107 cubic metres per 20 kg bag is the usual figure for standard pre-mixed concrete. A 40 kg bag is roughly double that.
But yield is a product characteristic, not a constant. Rapid-set mixes, high-strength mixes and post-setting products all differ, and every manufacturer prints their own yield on the bag. If the job is large enough for the difference to matter, read the bag rather than trusting a general figure — including this one.
Yield also assumes the mix is made to the stated water ratio. Wetter concrete is easier to place, weaker when cured, and does not stretch further.
Where the crossover sits
Somewhere between 0.2 and 0.5 cubic metres, and the reason is time rather than money.
Below about 0.2 m³ — a handful of post holes, a small pad — bags win. No delivery to book, no minimum charge, and you mix what you need when you need it.
Above roughly half a cubic metre the arithmetic turns. That is around 47 bags, close to a tonne to carry, and mixing it batch by batch means the pour takes hours. Concrete stiffens as it goes, so a slab poured slowly in small batches ends up with cold joints running through it where one batch went off before the next arrived. Those are planes of weakness in a slab that is meant to be monolithic.
Most Australian suppliers have a minimum delivery around one cubic metre with a short-load charge below that. Even paying the short-load fee, a single continuous pour is usually the better outcome for anything structural.
Allow for waste, and prepare the base
Five to ten per cent on a flat, well-prepared slab. More for hand-dug trenches, where the depth varies more than the drawing suggests.
That allowance is not padding. Concrete is lost to spillage, to an uneven sub-base that is deeper than nominal in places, and to the simple fact that a barrow cannot be scraped completely clean.
Screeding the base to level and compacting it does two jobs: it stops the volume creeping past the estimate, and it gives the slab uniform support. A slab sitting on soft patches cracks over them regardless of how good the mix was.
Where these numbers come from
- Australian pre-mixed concrete bag yield — manufacturer product data ≈0.0107 m³ per 20 kg bag; confirm against the product you buy
- Cement Concrete & Aggregates Australia Industry guidance on concrete density and placement
Method last reviewed 2026-10-28 This guide is re-checked against the sources above at least once a year, and again whenever a rule it explains changes. Out of date? Tell us and we will fix it.