Sizes for Large Envelopes and Flats

The Postal Service 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.

There are special size requirements for mailing flats at automation prices and carrier route prices. Contact your Business Mail Entry Unit for more information.

  1. Sizes for postcards
  2. Sizes for Letters
  3. Sizes for parcels that weigh 13 ounces or less.