Concrete for a 12×20 slab at 4" thick
A 12 ft × 20 ft slab poured 4 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
3.26cubic yards
88
cubic feet
147
80 lb bags
196
60 lb bags
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For a slab this size, 3.26 cubic yards is usually cheapest as ready-mix delivery. About 147 bags of 80 lb mix would do it by hand.