How many lbs bags needed to do a 12'x12' slab?

To calculate the number of pounds of concrete bags needed for a 12' x 12' slab that is 4 inches thick, first determine the volume in cubic feet. The slab volume is 12' x 12' x (4/12)' = 12 cubic feet. Since a typical 80 lb bag of concrete yields about 0.6 cubic feet, you'll need approximately 20 bags (12 cubic feet ÷ 0.6 cubic feet per bag = 20 bags).