Concrete for a 20×30 slab at 6" thick
A 20 ft × 30 ft slab poured 6 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
12.22cubic yards
330
cubic feet
550
80 lb bags
734
60 lb bags
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For a slab this size, 12.22 cubic yards is usually cheapest as ready-mix delivery. About 550 bags of 80 lb mix would do it by hand.