The size of the envelope used for USPS mail should properly accommodate the contents being mailed. Use light colored envelopes for best legibility. Avoid brilliant, deep colors.
In the USPS, there are two categories of envelopes: letter size and flats. Each category is rated differently by the USPS. To ensure the best rate for your mailing, call Mail Services for size and rate information prior to mailing.
Letter-Size Envelopes - In order to receive USPS automated processing, letter-size envelopes MUST:
6 1/8 inches
In addition, it is recommended that all envelopes or cards:
NOTE: Maximum thickness for automated processing is 1/4 inch. Mail that does not meet automated processing size may not qualify for discounted rates. Letter size envelopes (up to 6 1/8 inches x 11 1/2 inches) over 1/4 inch thick will be returned to the originator for re-enveloping.
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 or blue border which ensures proper posting of international rates, avoiding possible return for insufficient postage. Since the red and blue bordered envelopes indicate air mail to a foreign country, they should not be used for mailings within the United States.
Plain White or Manila Envelopes
These may used for all classes of mail. They should, however, be marked to show proper class of mail desired, such as first-class or standard.
These envelopes are to be used ONLY for first-class mail.
USPS "Flats" - Flats are envelopes larger than the maximum letter size, but not larger than 12 inches high by 15 inches long, and 1.5 inches thick.