Is there a magic number that I can multiply X my sqft. to get how much concrete I need to buy?

A cubic yard is 27 cubic feet or 3ft X 3ft X 3ft.

To determine how many cubic yards of concrete you need, you first must measure the area for the concrete in width by length.

If you have an area that is 10 feet wide and 20 feet long you have an area of 200 square feet.

If this area was 1 foot thick you would have 200 cubic feet.

But most concrete is poured in either 4" or 6" depths.

To determine how many cubic feet divide the 200 square feet by 3 for 4" thickness. (4" is 1/3 of a foot) This will give you 66.66 cubic feet. Divide 66.66 by 27 to determine cubic yards= 2.47 cubic yards of concrete.

For a slab 6" thick divide 200 by 2 (6" is 1/2 a cubic foot) for 100 cubic feet. Divide this by 27 to get 3.70 cubic yards.