Concrete is ordered by the cubic yard . Measurements need to be in the same unit, i.e. feet, so you need you convert the yards and inch numbers. Since the volume of a cube is the product of the side measurements, a cubic yard is 27 cubic feet. Now find out how many cubic yards your patio needs by multiplying the dimensions (in feet) of your patio. Change 4 inches to 1/3 foot and multiply 15 x 11 x 1/3 and you get 55 cubic feet. To determine the number of yards to order, you divide the cubic feet of your patio (55) by the cubic feet in each yard (27) and find that you need 2.037 cubic yards.