First-Class Mail
First-Class Mail® is used for personal and business correspondence. It includes postcards, letters, large envelopes, small packages, and any mailable item weighing 13 ounces or less. Pieces over 13 ounces can be sent as Priority Mail.

The minimum size for First-Class Mail is 5 inches long, 3-1/2 inches high, and 0.007 inch thick. All First-Class Mail nonmachinable letters are subject to the 17-cent nonmachinable surcharge.

For more details on postcard and stamped card rates, postcard rate dimensions, and the nonmachinable surcharge, refer to the Domestic Mail Manual.