Not all postage stamps are the same size. Traditional first-class stamps are usually less than a square inch in size, and many have the same dimensions.
1 First Class Forever Stamps
The most common type of U.S. Postal Service stamps you will encounter is the first-class "forever stamps," which you can buy and use at any time in the future to send a standard size letter, even if the price of postage goes up. They can be used to mail letters weighing up to one ounce.
Those stamps are most frequently 0.87 inches by 0.979 inches, but they can vary in size. Stamps are often displayed in post offices so you can see which ones are a convenient size, and you can also order stamps online or over the telephone. When the USPS issues a new stamp design, such as commemorative stamps, the announcement will often include the dimensions of the stamp. You can also contact the USPS to inquire about ordering stamps of particular denominations and, if needed, ask about the sizes.
Items that are unusually sized, or in some way difficult for the USPS's sorting equipment to handle, might cost more than a forever stamp to mail. You can measure and weigh an item at home or at your place of business to determine what category it fits. If you have questions, you can take an item to a post office and ask how much it costs to send, and they can sell you the appropriate stamp and help you ship the letter or package. Your letter carrier might also be able to tell you if you can send an item with a single stamp.
2 Other Types of Stamps
The First Class rate only applies to standard-sized letters under an ounce. Mailing other types of items, such as packages, can be more expensive. Mailing certain types of mail, particularly postcards, can be cheaper.
You can buy stamps in a variety of different values to suit your mailing needs. If you are mailing a letter or package that requires multiple stamps due to its weight or size, you should ensure that you are using a selection of stamps that fit the package, leaving room for your return address and the address where you are sending the package. In some cases, packages might be eligible for discount rates, like when shipping media items such as books and DVDs.
You can also send some items in flat rate packaging, where you are not charged according to the weight of what's inside. Since various factors impact the cost of mailing different items, including weight, speed and other services like delivery confirmation, you might want to take packages to a post office or third-party mailing store to determine your options before buying stamps.