Sizes and Types

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 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 mail piece. Use light colored envelopes for legibility, and avoid bright colors.

Letter Size

Letter size mail must be rectangular in shape for automated processing by USPS.

If the mail exceeds the following dimensions, the postage rate increases even if the mail weighs under one ounce:

Height Length
3 1/2 in. (min) 5 in. (min)
6 1/3 in. (max) 11 1/2 in. (max)


Flat Size

Flats are envelopes larger than the maximum letter size, but no larger than 12" high by 15" long and 3/4" thick. 


These envelopes contain a cushioned lining to provide a degree of safety for mailing small and fragile merchandise. These are available at the Campus Book Store. 


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.