The 6 5/8 Booklet envelope has a width of 9.5 inches and a height of 6 inches, giving it a total surface area of 57 square inches. Its maximum enclosure size is 5.75 x 9.25 inches. This booklet envelope is sometimes referred to as the 6-5/8 catalog envelope, due to its dimensions.
Booklet envelopes like the 6 5/8 get their sizing from the height and width in inches rather than a numbering system. The 6 5/8 booklet has an open side seam allowing easy insertion of flat contents without folding. The "5/8" indicates this envelope is slightly taller than the 6 1/2 booklet. The extra height provides more room for documents, booklets, pamphlets and other products benefiting from flat, protected mailing. With its open side and space between the 6 1/2 and 9 booklet sizes, the 6 5/8 booklet envelope serves an incremental niche for enveloping flat mailings.