Concrete for a 30×40 slab at 6" thick
A 30 ft × 40 ft slab poured 6 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
24.44cubic yards
660
cubic feet
1100
80 lb bags
1467
60 lb bags
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For a slab this size, 24.44 cubic yards is usually cheapest as ready-mix delivery. About 1100 bags of 80 lb mix would do it by hand.