What is the maximum thickness of a letter-size mailpiece?

The maximum thickness of a letter-size mailpiece is set at ¼ inch. This measurement is crucial for ensuring mail is classified and processed correctly within postal systems. When a mailpiece exceeds this thickness, it transitions from being classified as a letter to being categorized under other mail types, such as flats or packages, which often incurs different postage rates and handling procedures.

Understanding this standard is essential for anyone dealing with mailing practices, as it helps avoid issues with mail acceptance and delivery speed. The choice of a thickness surpassing this limit would lead to additional postal fees or could result in delays, since the mail may require different handling and sorting processes. Hence, keeping your letter-size mailpieces at or below the ¼ inch threshold ensures compliance with postal regulations.