You can buy pre-mixed bags of ready-to-mix concrete. These simply require water and mixing; the measuring is already done for you. An 80 lb bag (on average) will make 4 sq. feet x 2.5" thick. A more economical method is to make your own concrete, using this simple time-tested formula as a guide - 1:2:3 parts by dry volume - one part Portland cement, two parts sand, three parts coarse aggregate.