What is the most commonly used envelope size for mailing letters?

The No. 10 envelope is widely recognized as the standard size for mailing letters, particularly in business and professional contexts. This envelope size measures 4 1/8 inches by 9 1/2 inches, making it ideal for holding a standard letter that is typically printed on standard 8 1/2 x 11-inch paper, folded into thirds. Its common use is attributed to its compatibility with letterhead and easy fit in most mailboxes, which facilitates efficient handling by mailing systems.

While other envelope sizes, such as No. 9 and No. 12, exist, they serve different functions. The No. 9 envelope, for instance, is often used for reply envelopes included with correspondence as it fits neatly within the No. 10 envelope. The No. 12 and No. 8 envelopes are less frequently used for standard letters and are typically reserved for specific uses such as larger documents or personal correspondence, respectively. Hence, the No. 10 envelope's prevalence in business communication attests to its practicality and suitability for a wide range of mailing needs.