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:

* Large Envelopes exceed one or more of the letter size minimum 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[1] – the higher the weight the higher the postage, especially for First-Class Mail.

More about Physical Standards for Commercial Flats[2]

Is it possible to fold your flat-size piece to letter-size[3]? If so, you can save money because you'll pay the lower letter price.

TIP
--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.
-Standard Mail Carrier Route flats.

There are special size requirements for mailing flats at automation prices[4] and carrier route prices[5]. Contact your business mail entry unit[6] for more information.

Sizes for cards[7]

Sizes for letters[8]

Sizes for parcels[9]