When I purchased this item, I already owned a Calphalon 12x17 "Jelly Roll Pan" and a Silpat E04 11.5x16.5 mat. The Jelly Roll Pan is good quality and actually measures 11.5x16.5 internally at the bottom, and the Silpat fits perfectly. I have had the occasion to want a deep baking pan with the Silpat and was delighted to see that Calphalon makes a 12x17 deep baking pan. I presumed that the two 12x17 pans would be the same size. When I received it, I immediately found that the Silpat would not fit and that the inside bottom of this pan is closer to 11.25x16.75. I'm not sure if this is a standard baking nominal size, but I will be returning the baking pan.