Almost every slab that cracks was too thin, or was poured on ground that moved. The concrete is the easy part. This works out the thickness for what the slab actually carries, then the volume — and lists the things that are not in that number.
Pick what the slab is for and the thickness follows. Then enter the size.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Almost every slab that cracks was too thin, or was poured on ground that moved. Pick what the slab is for and the thickness follows — around 100 mm for a shed base, patio or path, more for anything taking vehicles — then the volume is area × that thickness, plus 10% for spillage and an uneven subgrade. A 3 × 3 m shed base at 100 mm is 0.9 m³, so about 1 m³ to order.
Concrete is strong in compression and weak in tension, so it cracks when the ground beneath it moves, not usually when the load is too great. A well-compacted sub-base under a 100 mm slab outlasts a 150 mm slab poured on soft ground. Spend the effort below the slab before spending money on depth. And do not pour onto frozen ground or leave it uncovered with frost forecast — concrete that freezes before it sets is permanently weakened, with no repair short of breaking it out.
Reinforcement laid on the sub-base and poured over does nothing at all, because it is not in the part of the slab that is in tension. Mesh belongs in the middle of the depth, on proper spacers. And anything larger than about 25 m² wants movement joints: concrete shrinks as it cures and will crack somewhere, so a saw-cut or formed joint about a quarter of the slab depth decides where. Point loads — a jack stand, a post base, a machine foot — are a separate question the thickness above does not cover.
Around 100 mm for a shed base, patio or path over well-compacted ground; more for anything carrying vehicles. But the sub-base matters more than the thickness: a 100 mm slab on properly compacted ground outlasts a 150 mm slab on soft ground.
In the middle of the slab depth, on spacers. Mesh laid on the sub-base and poured over does nothing, because it is not in the part of the slab that goes into tension.
In anything larger than about 25 m2, yes. Concrete shrinks as it cures and will crack somewhere — a joint decides where. Saw-cut or form them to roughly a quarter of the slab depth.
Not onto frozen ground, and not without covering it if frost is forecast. Concrete that freezes before it sets is permanently weakened and there is no repair short of breaking it out.