Too easy - 18" by 26" as measured across the top. Because the sides slope in toward the bottom, the bottom measures 17-3/4" by 25-3/4".

If you are asking to see if you can possibly jam one into your home oven, the answer is no, especially since you want 2 inches free on each side of the pan to allow the hot air to circulate freely. There is a 3/4-size pan, 15" by 21", that will fit most home ovens; half-size pans, at 18" by 13," coming out your ears; and many choices for quarter-size pans, as well, at 13" by 9".