Concrete for a 20×30 slab at 5" thick
A 20 ft × 30 ft slab poured 5 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
10.19cubic yards
275
cubic feet
459
80 lb bags
612
60 lb bags
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For a slab this size, 10.19 cubic yards is usually cheapest as ready-mix delivery. About 459 bags of 80 lb mix would do it by hand.