US Post Office Size Requirements

For letters, postcards, and flats

Letter-size Dimensional Template - click to download the template

Sizes for Letters

To be eligible for mailing at the price for “letters,” a piece must be:
  • Rectangular
  • At least 3-1/2 inches high x 5 inches long x 0.007 inch thick
  • No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick
  • What is high? What is long?
    • Length is the side parallel to the address
    • Height is the side that is perpendicular to the length

First-Class Mail or even Standard Mail “letters” that have one or more nonmachinable characteristics will pay a nonmachinable surcharge. Customers can be unpleasantly surprised that they must pay extra postage when, for example, they mail a square greeting card. The post office charges extra postage because mailpieces that are rigid, square, or unusually shaped often jam postal equipment and are difficult to process. This costs the post office time and money — and may also damage your mailpiece.

Folded Pieces

Many mailers fold sheets of paper to form letter-size pieces or newsletters. Folded pieces, or folded self-mailers, can save time and money because you're not paying for or stuffing envelopes. Folded self-mailers must be sealed or they will be subject to the nonmachinable surcharge. We have a tabbing machine that uses USPS approved wafer seals, also called “tabs”, to close the open sides of folded mailpieces.

The fold is requested to be on the bottom (the side below the address) and secure the opening at the top with a piece of tape, a tab, or a wafer seal.


  • 0.007 inches? How do I measure that? As a guide, an index card is thick enough.
  • What is high? What is long?
    • Length is the side parallel to the address
    • Height is the side that is perpendicular to the length

Sizes for Postcards

Postcards are an inexpensive way to get an immediate message to customers. When they arrive in the mail, there's the message — no envelope to open! First Class Mail postcards are a great value, too. With First Class Mail postcards, you pay a low price and get all of the benefits, like forwarding and return that come with First Class Mail. And, if you mail with single-piece First Class Mail postage affixed, there is no extra work involved — simply drop the postcards in a collection box.

Postcard requirements

You may think that your mailpiece is a "postcard," because it is a single sheet of paper. But to qualify for mailing at the First Class Mail postcard price, it must be:

  • Rectangular
  • At least 3-1/2 inches high x 5 inches long x 0.007 inch thick
  • No more than 4-1/4 inches high x 6 inches long x 0.016 inches thick

Sizes for Large Envelopes and Flats (EDDM)

The post office uses the word "flats" to refer to large envelopes, newsletters, and magazines. The words “large envelopes” and “flats” are used interchangeably.

Whatever you call them, flats must:
  • Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick
  • Be no more than 12 inches high x 15 inches long x ¾ inch thick

Dimension Minimum* Maximum
Height 6-1/8 inches 12 inches
Length 11-1/2 inches 15 inches
Thickness 1/4 inch 3/4 inch

* Large Envelopes exceed one or more of the letter size maximum dimensions.

The maximum size for a flat provides plenty of room to put lots of material in the envelope. But keep in mind that weight usually affects price — the higher the weight, the higher the postage, especially for First Class Mail.

More about Physical Standards for Commercial Flats

Is it possible to fold your flat-size piece to letter-size? If so, you can save money because you'll pay the lower letter price.

  • What is long? What is high?
    • For flats, "long" (the length) is the side parallel to the address as read.
    • "High" (the height) is the side perpendicular to the length.
  • This general rule does not apply to:
    • Automation flats
    • USPS Marketing Mail Carrier Route flats

** Note if the hyperlinks are not working, it could be because the post office has changed their site pages, if you could let us know, then we can fix it for you! Thank you!

Click to download how to use the template


I would like to express our appreciation to Becky and Scott and the staff at Print & Copy Factory for their assistance in creating our new website. They had all the tools we needed to build the site we wanted, and then trained us on how to add content to it. Everything is now easy to change pages and add content.
Operations Manager, North Sound Alarm

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