What is the minimum size envelope that can be mailed via the United States Postal Service?

The minimum size envelope that can be sent through the United States Postal Service is 3.5 inches high, 5 inches long and 0.007 inches thick. It has to be rectangular in shape and weigh 3.5 ounces or less.

Odd-shaped envelopes or those whose contents are lumpy, rigid or more than .25 inches thick are accepted but incur a nonmachinable surcharge. Putting the address parallel to the short end of the envelope or using an envelope with a clasp can also trigger the surcharge. Envelopes whose length divided by height is less than 1.3 or more than 2.5 are considered odd shaped. If a smaller or unusual shape is preferred, a package can be used, as these have no minimum size limitations.