How many cubic yards of concrete — or how many 80-lb bags — for your slab.
How this worksVolume is length × width × (thickness ÷ 12) in cubic feet, divided by 27 to get cubic yards — rounded up to two decimals, since a short load is a problem. Each 80-lb bag of premixed concrete yields about 0.6 cubic feet, so bags are total cubic feet ÷ 0.6, rounded up to a whole bag. For pours over about 1/2 cubic yard, a ready-mix truck is usually cheaper than bags.