Concrete for a 30×30 slab at 6" thick
A 30 ft × 30 ft slab poured 6 inches thick needs the amounts below (a 10% waste allowance is already included).
Concrete needed
18.33cubic yards
495
cubic feet
825
80 lb bags
1100
60 lb bags
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For a slab this size, 18.33 cubic yards is usually cheapest as ready-mix delivery. About 825 bags of 80 lb mix would do it by hand.