How many stamps do you put on a 9x12 envelope?

The United States Postal Service classifies a 9-inch by 12-inch envelope as a “large envelope” or a “flat.” Mailing such an envelope requires two first-class postage stamps for the first ounce and additional postage for each additional ounce. Large envelopes that are thicker than 3/4 inch require package-rate postage.

Any envelope larger than the maximum dimensions of a U.S. postal letter (6 1/8 inches high, 11 1/2 inches long and 1/4 inch thick) is classified as a “large envelope” and requires extra postage. Maximum dimensions for large envelopes or flats are 15 inches long by 12 inches wide by 3/4 inch thick. A flat-rate envelope is another choice for mailing heavy documents that require faster delivery.