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The normal thickness of concrete in driveways is 4 to 5 inches. Using a thickness of 5 inches, here is the yardage:
18 ft x 21 ft x 5/12 ft = 157.5 cubic feet
Since 1 cubic yard = 3'x3'x3' = 27 cubic feet
Hence, the 157.5 cubic feet is equal to 157.5/27 cubic yards = 5.8 cubic yards.
Allowing for waste, you should order 6 yards of concrete.
Hope this answers your question.