Please note: For your convenience, this calculation has been rounded up slightly.
Important reminder: Every effort has been made to ensure the accuracy of this calculation. But before purchasing materials for any project, please check amounts with your suppliers or contractors.
Check with your building department before beginning any concrete project. Local codes will specify how deep a slab or footing must be, as well as how much gravel to put in the bottom of the excavation before pouring the concrete. In many cases, a specific type of metal reinforcement -- either wire mesh or reinforcing bars -- will be required. An inspector may want a sample of the concrete, to make sure it will be strong enough.
A large slab is usually a job for professionals. Any mistakes will indeed be "set in concrete." Finishing the concrete surface is a difficult job requiring special skills and near-perfect timing. If you tackle a project that calls for a smooth concrete surface, be sure that you have on hand an experienced finisher.
A few tips: When excavating for concrete slabs and footings, scrape the sides and bottoms of the holes smooth. If the sides will be raised above the ground, make forms for the sides using 2x lumber and stakes made out of 2x4 or 2x2.
If you need 3/4 of a yard or more of concrete, it usually makes sense to order it delivered in a concrete truck. For smaller amounts, you might do better mixing your own, either in a rented rotary mixer or in a wheelbarrow. Some concrete companies have special on-site mixing trucks designed for small deliveries.
Before the truck arrives, have everything prepared -- all the forms firmly in place, the gravel laid and tamped, and the reinforcing metal in position. Have two wheelbarrows with able-bodied helpers ready to go. Make sure you know just where the truck will park. Lay 2x8s or 2x10s on the ground to serve as paths for the wheelbarrows. Post one person at the truck, to regulate how much concrete flows into the wheelbarrows. At least two people are needed at the excavation, to spread and screed the concrete, and later to finish it.