What are the dimensions of a standard size letter for mailing purposes?

A standard size letter for mailing purposes in the United States typically measures 8.5 x 11 inches (21.6 x 27.9 cm). This size is commonly used for business correspondence, personal letters, and documents. When folded, it fits neatly into a standard #10 envelope, which measures 4.125 x 9.5 inches. This format is widely recognized and accepted by postal services for efficient processing.