Letter Types:

• The size of the envelope should properly accommodate the contents. For use in the U.S. Postal System there are basically two categories of envelopes: letter size and “flats”, as referred to by the U.S. Postal Service.
• Letters and non-letters or “flats” are rated differently by the USPS. To assure the best rate, call Mail Services for size and rates before preparing the mailpiece.
• DO NOT use red envelopes. Other dark colored envelopes may also cause problems with USPS address scanners.

Letter Size:

• Letter size mail must be rectangular in shape for automated processing by USPS.
• To ensure prompt and efficient automated processing of First Class mail, all envelopes and cards must:
  • Have an aspect ratio of width (height) to length between 1 to 1.3 and 1 to 2.5 inclusive (length divided by width). Ask Mail Services if you are not sure of sizing.
  • The following dimensions must be met for automated processing by USPS:Height: 3 1/2″ Min 6 1/8
    Max Length: 5″ Min 11 1/2″
    Max Thickness* .007 Min 3/16″ Max* Firmly compressed ** Card Stock should not exceed .0095″

Flat Size:

• Flats are envelopes larger than the maximum letter size:
  6-1/8″ high x 11-1/2″ long x 1/4″ thick, but not larger than 12″ high x 15″ long x 3/4″ thick.
• Address flat envelopes so the flap is to the right edge, back side, for machine processing.
• Priority – Should be used for First Class mail weighing over 13 ounces and other mail under 13 ounces requiring expedited delivery. (Priority red, white, and blue envelopes are available from Mail Services free of charge.)

Strength:

• The strength of the envelope must be such to protect and retain the contents. The use of oversized envelopes or overstuffed envelopes can damage or cause the loss of the contents. Heavy materials may require TyvekTM envelopes. Do not secure with string.

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