The #8 5/8 Business envelope has a width of 8.625 inches and a height of 3.625 inches, giving it a total surface area of 31.26 square inches. Its maximum enclosure size is 3.375 x 8.375 inches. This envelope is also referred to as the 8-5/8 envelope, corresponding to its height dimension.
The #8 5/8 gets its numbering from William Irwin Martin's late 1800s system that standardized envelope sizes. The "5/8" indicates this envelope is slightly shorter in height than a standard #9 envelope. The #8 5/8 became popular for commercial mailings needing more vertical space than smaller envelopes provide. Its dimensions work well for brochures, catalogs, magazines and other mailings that benefit from the extra height. While not as ubiquitous as sizes #6-#10, the #8 5/8 fills a niche for mailings needing roomier dimensions without going full #9 size. It continues to serve business mailing needs when an incrementally larger size is required.