What Is the Standard Letter Size For Mailing? 

When it comes to sending mail, there are many different sizes and styles that can be used. Some of them are more effective for certain types of applications than others. But no matter what type of envelope you need to send your mail, you should always use the correct size. This will ensure that your mail gets to its destination safely without costing you any extra money. 

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Business Envelopes

Regular envelopes are typically the sizes that you see most often for commercial mailings, such as the standard #10, which measures 4 1/8 inches by 9 1/2 inches. This size is ideal for invoices, letters, and other business correspondence. In addition, these envelopes come in a wide variety of styles and colors. 

Catalogs and Other Packages

When sending a large amount of material, it can be tempting to choose a larger envelope than is needed. This can result in a lower postage price, but it can also be difficult to enclose everything properly and protect it from damage while traveling in transit. 

The best way to decide on the right envelope for your project is to determine how it will be used and the expected contents. This will help you to select the correct envelope type and determine which sizes and styles are best for each purpose. 

Aside from choosing the proper envelope, you should also consider the weight of your piece and how it will be mailed. Heavy materials will cost more to mail than light ones, and each additional ounce increases the price of postage. 


A flat-size piece is usually a sheet of paper that has been folded into a rectangular shape, but it can also be a roll or a sheet of corrugated paper. When mailing a flat, it is important to keep in mind that the longest side of the flat must be at least 3′′ long and 5″ high. 

It is also important to determine if the flat has finished corners. If the corners are not finished, it is not considered a flat. If the finished corners are more than 1/8′′ thick, it is considered a postcard or letter-size card-type mailpiece. 

Whether you need to mail a postcard or a letter, the USPS has some guidelines that will help you determine if it is a mailpiece that will qualify for the appropriate postage rates. First-Class Mail (formerly known as Standard Mail) postcards are an inexpensive and quick way to reach customers. They are available in a variety of sizes and can be mailed at First-Class Mail prices. 

Mailpieces mailed at Marketing Mail rates are called “letters.” They do not receive a postage break, but they do receive a lower price than postcards. They are also more versatile than postcards and can be mailed anywhere in the world. 

The most common postal sizing standards apply to mailpieces that are mailed via First-Class Mail service and USPS Marketing Mail rates. If you have any questions about what is the standard letter size, contact your local post office and speak with a customer service representative.