Flats (USPS)

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The Postal Service uses the words "flats" and "nonletters" to refer to large envelopes, newsletters, and magazines. The words are interchangeable. Whatever you call them, flats must:

Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long (the side parallel to the address as read) OR ¼ inch thick.
Be no more than 12 inches high x 15 inches long x ¾ inch thick.

NOTE: This general rule does not apply to:

Automation rate flats.
Standard Mail Enhanced Carrier Route flats.

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 rate) – the higher the weight the higher the postage, especially for First-Class Mail.

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 rate.