Concrete, as you probably know, is sold in cubic feet. Make sure you have the proper base. Also, consider steel re-inforcement. (Fiberglass reinforced concrete may NOT be enough depending on the weight and span of the patio.)
If you want to pour a patio that's 15' x 12' x 4", then the conversion to yards is 15' x 12' x .333 feet = 59.4, divided by 27. (There are 27 cubic feet in 1 cubic yard.) 59.4 cubic feet divided by 27 = 2.2 cubic yards.