A letter can weigh up to 1 ounce and still be eligible for a first-class stamp from the United States Postal Service (USPS). The USPS defines a letter as a piece of mail that is rectangular in shape, has one dimension that is at least 6-1/8 inches long and at least 3-1/2 inches high, and is no more than 11-1/2 inches long and 6-1/8 inches high. A letter must also be flexible and not contain any items that are not permitted in the mail, such as hazardous materials or live animals.
If a letter weighs more than 1 ounce, you will need to pay an additional fee for each additional ounce. As of January 2021, the price of a first-class stamp for a letter that weighs 1 ounce is $0.55. For each additional ounce, you will need to pay an additional $0.20, for a total of $0.75 for a 2-ounce letter, $0.95 for a 3-ounce letter, and so on.
It’s important to note that the USPS also offers other shipping options for larger or heavier items that do not meet the requirements for a letter. These options may include flat-rate envelopes, padded envelopes, or packages. You can check the USPS website or contact your local post office for more information about these options and the fees associated with them.