The average cereal box is about 12 inches high and 8 inches wide. Not every cereal box is the same size, but most are within an inch of each other, depending on what’s inside.
Kellogg’s, which makes some of the most popular cereals, mostly sells them in boxes that are 12 inches by 7 3/4 inches and about 2 inches deep. A brand’s boxes are also different sizes, in addition to having different kinds of cereal. Many brands also offer special deals when they make their box a certain amount bigger, usually about 25 percent bigger than usual.