Concrete volume and mass for a slab or footing.
Multiply length by width by thickness, all in metres. A 5 × 4 metre slab 10 centimetres thick is 5 × 4 × 0.1, or two cubic metres. In practice order about 5% more, since the base is rarely perfectly level and some concrete stays in the mixer.
The common mix is one part cement, two parts sand and four parts gravel, with water at about half the weight of the cement. One cubic metre takes roughly 300 kilograms of cement, 600 of sand and 1,200 of gravel.
A footpath is fine at 8–10 centimetres, a driveway or garage needs 12–15, and heavy machinery more. Every slab needs a compacted gravel base beneath it; without one, even a thick slab cracks in the first winter.
With the usual mix, one cubic metre takes about twelve 25-kilogram bags of cement. For a stronger concrete the cement share goes up, but the water must come down with it, or the strength does not rise and the cracking does.