Answer:
Figure about 1/2 cubic foot per 60 lb bag. The basic formula is to use equal amounts of cement, sand, and gravel. There are 27 cubic yards per cubic foot. It depends upon how thick you are planning to pour the concrete. For one foot thick, you would need 9 cubic feet of concrete. For 6" thick, you would need 4 1/2 cubic feet. I'll give you the answers for 1 ft thick and for 6" thick.
For 1 foot thick, you need 9 cubic feet at 1/2 cubic foot per 60 lb bag equals 18-60 lb bags.
That's six 60 lb bags of concrete, plus six 60 lb bags of sand, plus 360 lbs of gravel.
For 6 inches thick, divide each of these by 2 to give 3-60 lb bags of concrete, 3-60 lb bags of sand, plus 180 lbs of gravel.