Concrete for a 20×20 slab at 5" thick
A 20 ft × 20 ft slab poured 5 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
6.79cubic yards
183
cubic feet
306
80 lb bags
408
60 lb bags
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For a slab this size, 6.79 cubic yards is usually cheapest as ready-mix delivery. About 306 bags of 80 lb mix would do it by hand.