A slab 4 metres by 3 metres at 100 millimetres thick is 1.2 cubic metres of concrete. With a 10 per cent allowance for uneven ground and spillage that becomes 1.32 cubic metres, which weighs about 3.17 tonnes and works out at 143 bags of twenty kilogram pre-mix.
The volume sounds trivial. The bag count usually decides the job.
Why the allowance is not optional
Sub-grade is never perfectly flat, formwork flexes, and some concrete is lost to the mixer, the barrow and the ground. Ten per cent is a common starting allowance, and it is a floor rather than a ceiling on rough ground.
Running short mid-pour is the specific failure worth avoiding, because the first concrete placed begins curing immediately. A cold joint through the middle of a slab is a permanent weakness, not a cosmetic mark, which is why over-ordering slightly is standard practice.
What a mix ratio actually specifies
A ratio like 1:2:4 gives parts of cement, sand and aggregate by volume. For a cubic metre of finished concrete at that ratio, with a 5 per cent allowance, the materials come to 216 kilograms of cement, 480 kilograms of sand and 900 kilograms of aggregate.
The parts do not add to the finished volume, because the smaller particles fill the gaps between the larger ones. This is why you cannot work out a batch by adding up the parts, and why the concrete mix ratio calculator takes the target volume and works backwards to the ingredients.
Where the crossover sits
Below roughly a third of a cubic metre, bags are usually easier: no delivery window, no minimum order, no waste if the job stops. Above about a cubic metre, mixing by hand becomes a serious amount of labour, and consistency between batches starts to vary in ways that matter structurally.
At 143 bags, the slab above is well past the point where a truck or a trailer mixer is the sensible answer. Ready-mix also arrives at a specified strength grade, which hand mixing can only approximate.
The weight is worth knowing in advance
At about 2.4 tonnes per cubic metre, concrete is heavy enough that access and load limits become real constraints: 3.17 tonnes is more than most trailers and many domestic driveways are designed for.
The concrete volume calculator gives volume, bag count and weight together, and the concrete weight calculator covers the load question on its own.