14 inch single-layer cake and 14 inch double-layer cake are not the same size. Single-layer 14 inch cake because the size of the cake refers to inches, one inch equals 2.54 cm, and 14 inch is 35.5 cm. The size difference between the upper and lower layers of the double layer must be at least 4 inches to support the weight of the upper layer cake. So 14 inch double layer cake, the top layer must be 10 inch. 14 inch plus 10 inch is 24 inches.