Concrete calculator
Concrete bags list a yield in cubic feet, and that yield is what turns slab dimensions into a shopping list.
Bag yields, and the point where bags stop making sense
A 60 pound bag yields about 0.45 cubic feet and an 80 pound bag about 0.6 — the numbers used here. Those yields are why the bag count climbs so fast: a modest 10 by 10 foot slab at four inches thick is over 12 cubic feet, which is around 28 sixty-pound bags, or roughly 1,700 pounds of material to carry and mix.
That is the practical crossover. Past about a cubic yard, ordering ready-mix delivered is usually cheaper, and it is certainly easier, because a large slab poured in bag-sized batches risks cold joints where one batch stiffens before the next arrives. The cubic yard figure above is there so you can price both options.
The waste allowance covers uneven subgrade, spillage, and the fact that nobody screeds perfectly level. Ten percent is standard; go higher if your ground is rough, because the low spots take more than you think.
Questions
When should I order ready-mix instead?
Around one cubic yard, which is roughly 60 sixty-pound bags. Beyond that the cost, the labour and the risk of cold joints between batches all favour a delivery.
How thick should a slab be?
Four inches suits patios and walkways. Driveways and anything carrying vehicles usually want five to six inches, often with reinforcement — check local requirements.
Does the calculator include the base layer?
No. Gravel base under the slab is a separate volume at its own depth — use the gravel calculator on this site for that part.