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Sizes and Types

The size of the envelope should properly accommodate the contents. For use in the US Postal System there are basically three categories of envelopes: letter size, flat size and parcel size, as referred to by the U.S. Postal Service. Letters, flats and parcels are rated differently by the USPS. To assure the best rate, call Campus Mail Services for size and rates before preparing the mail piece. It is always more postage cost effective to package in the smallest type of envelope as long as you don’t exceed a dimension criteria for that envelope usage. Please refer to http://pe.usps.com/text/qsg300/Q201.htm#1009536 for letters, http://pe.usps.com/text/qsg300/Q301.htm#1009536 for flats and http://pe.usps.com/text/qsg300/Q401.htm#1009536 for parcels.

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.

3.1 Basic Standards for Automation Letters and Cards

Letters and cards claimed at automation letter prices unless prepared as a self-mailer, booklet, or postcard each must be a sealed envelope (the preferred method) or, if un-enveloped, must be sealed or glued along all four sides. All references to paper basis weight are based on measured weight for 500 17-inch by 22-inch sheets. Envelopes, or letter-size sealed along all four sides, must be made of paper with a minimum 16 pounds basis weight or equivalent.

3.2 Dimensions and Shape Standards for Automation Letters

Each letter-size piece must be rectangular and:,/p>

For height, no more than 6-1/8 or less than 3-1/2 inches high.

For length, no more than 11-1/2 or less than 5 inches long.

For thickness, no more than 0.25 or less than

0.007 inch thick if no more than 4-1/4 inches high and 6 inches long; or

0.009 inch thick if more than 4-1/4 inches high or 6 inches long, or both.

For more information for folded self mailers, booklets, postcards and heavy letters use the following link: http://pe.usps.com/text/dmm300/201.htm#wp1079302

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. Seal the gummed flap. Add tape, if you wish, not staples. Avoid clasp closures.

Follow this link for more information on physical standards for flat size mail.


Envelopes and boxes are free of charge from the USPS for priority postage rates. The flat rate boxes and envelopes are your best choices.

This rate should be used for First Class mail weighing over 13 ounces and other mail less than 13 ounces that you request a non guaranteed expedited delivery. Just keep in mind that a priority rate does not give a guaranteed delivery date.


The strength of the envelope or box 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. Don't secure with string.

To save postage, use the right size envelope for the contents!

Priority Mail Express

Envelopes and boxes are free of charge from the USPS for Express postage rates. Consult with Mail Services as to whether the flat rate envelopes and boxes will actually result in better postage rates for you.