Standards for Lettersized Mail

To avoid the per-piece postal surcharge on “non-standard size” mail stick to the following: All mailpieces that are 1/4” thick or less must be rectangular, at least 5” long and at least 3 1/2” high. All mailable matter must be at least .007” thick, weigh no more than 70 pounds, and be no more than 108” in combined length and girth. Use this chart for quick size referencing:  

When Height of Envelope is Length of Envelope Minimum Must Be Between Maximum 3 1/2 5 8 3/4 3 5/8 5 3 1/16 3 3/4 5 9 3/8 3 7/8 5 9 11/16 4 5 3/16 10 4 1/8 5 3/8 10 5/8 4 1/4 5 1/2 10 5/8 4 3/8 5 11/16 10 15/16 4 1/2 5 7/8 10 15/16 4 1/2 5 7/8 11 1/4 4 5/8 6 11 1/2 4 3/4 6 3/16 11 1/2 4 7/8 6 5/16 11 1/2 5 6 1/2 11 1/2 5 1/8 6 11/16 11 1/2 5 1/4 6 13/16 11 1/2 5 3/8 7 11 1/2 5 1/2 7 1/8 11 1/2 5 5/8 7 5/16 11 1/2 5 3/4 7 1/2 11 1/2 5 7/8 7 5/8 11 1/2 6 7 13/16 11 1/2 6 1/8 7 15/16 11 1/2

To meet standard postage rate regulations the thickness of letter-size mail cannot exceed 1/4".
Flat-size Mail Standards
Minimum: more than 11 1/2” long, or 6 1/2” high, or more than 1/4” thick.
Maximum: 15” long, 12” high, and 3/4” thick.
May be wrapped, unwrapped, sleeved, or enveloped.
For further details on Basic Mailability and Processing Categories
consult U.S. Postal Service Quick Reference Guide (Publication 95).
 

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