Concrete for a 20×20 slab at 6" thick
A 20 ft × 20 ft slab poured 6 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
8.15cubic yards
220
cubic feet
367
80 lb bags
489
60 lb bags
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For a slab this size, 8.15 cubic yards is usually cheapest as ready-mix delivery. About 367 bags of 80 lb mix would do it by hand.