Types and Sizes


The size of the envelope should properly accommodate the contents. In the US Postal System there are two categories of envelopes: letter size and flats.

Letters, non-letters and barcoded flats are rated differently by the USPS. To assure the best rate, call Mail Services for size and rates before preparing the mail piece. Use light colored envelopes for legibility and please avoid brilliant colors.

Post Cards

The following dimensions are the maximum and minimum size for post cards.

Height: 3 1/2" Min 4 1/4" Max
Length: 5" Min 6" Max
Thickness .007" Min .016" Max

Letter Size

Letter size mail must be rectangular in shape for automated processing by USPS.
To ensure prompt and efficient processing of First Class mail, it is recommended that all envelopes and cards :

  • Have an aspect ratio of width (height) to length between 1 to 1.3 and 1 to 2.5 inclusive. See Mail Services for envelope template if you are not sure of sizing.
  • Be sealed or secured on all four edges so that they can be handled by machines.
  • Meet the following dimensions for automated processing by USPS:
Height: 3 1/2"Min 6 1/8" Max
Length: 5" Min 11 1/2" Max
Thickness* .007 Min 1/4" Max
*Firmly compressed **Card Stock should not exceed .016"

Note: Maximum thickness for automated processing is 1/4". Mail that does not meet automated processing size may not qualify for discounted rates. Letter size envelopes (up to 6 1/8" X 11 1/2") over 1/4" thick will be returned to originator for re-enveloping.

No letters (Flats)

Non letter size envelopes are larger than the maximum letter size, but not larger than 12" high by 15" long and 3/4" thick.

International Letter Size Envelopes

Mail in letter size envelopes for delivery to foreign countries should be placed in "International" Envelopes. These envelopes have a red and blue border which assures proper posting of international rates and avoid return for insufficient postage. Since the red and blue bordered envelope indicates air mail to a foreign country, these envelopes should not be used for mailings in the United States.

Green Bordered

Used only for First Class Mail.

Plain White or Manila

Can be used for all other classes of mail. These should be marked to show proper class of mail, i.e., First or Third Class.


These envelopes contain a cushioned lining to provide a degree of safety for mailing small and fragile merchandise.


Should be used for First Class mail weighing over 13 ounces and up to 70 pounds, or any other mail under 13 ounces requiring expedited delivery. Priority Mail provides two day service between major metropolitan areas. This service is not guaranteed.