Concrete for a 30×40 slab at 5" thick
A 30 ft × 40 ft slab poured 5 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
20.37cubic yards
550
cubic feet
917
80 lb bags
1223
60 lb bags
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For a slab this size, 20.37 cubic yards is usually cheapest as ready-mix delivery. About 917 bags of 80 lb mix would do it by hand.