A concrete garage floor is made up of cement, sand and aggregates. Cement is reckoned in bags per cubic yard, with more bags of cement yielding a stronger concrete. A bag of cement is one cubic foot of cement, or about 94 lbs. Calculating the quantity of cement required for a garage floor requires that you know the dimensions for the floor, and that you know how strong you want the floor of the garage to be. The Portland Cement Association recommends a minimum cement content of 6 bags per cubic yard of concrete.
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 Calculator

Use your calculator to multiply the length of the concrete slab in feet by the width of the concrete slab in feet. Multiply the result by the thickness of the slab in feet. The result is the number of cubic feet in the concrete slab. If your garage is 10 ft. by 27 ft. and the slab is 1 ft. deep, then, 27 x 10 x 1 = 270 cubic feet of concrete.

Divide the number of cubic feet in the concrete slab by 27. There are 27 cubic feet in one cubic yard. If you need 270 cubic feet of concrete for your slab, then, 270 / 27 =10 cubic yards of concrete.

Multiply the number of cubic yards by the strength you want the concrete to have. A "6bag" mix is stronger than a "4bag" or "5bag" mix. If you want a 6bag mix, then you will need six bags of cement per cubic yard of concrete: 6 x 10 = 60 bags of cement.