Concrete

Concrete is sold two ways and the choice matters more than the mix. Bags are convenient up to about a cubic yard, which is roughly forty-five 80 lb bags; past that you are mixing for most of a day and a ready-mix truck is both cheaper and better, because a slab poured in one go has no cold joints in it. Every calculator here works out the volume first and then converts it to whichever you are buying. The thing that catches people out is depth. Ground is never as level as it looks, excavations are deeper in the middle than at the edges, and a slab measured once at the corner will run over. That is why each of these carries a waste allowance you can see and change, rather than a hidden margin. Thickness, reinforcement and footing depth are regulated and vary by jurisdiction, so treat any figure here as a starting point and check your local building code.