Enclosures will determine the appropriate envelope to be used, meaning the envelope will be one of the right size and strength.

• The size should properly accommodate the contents. Excessively large envelopes will not keep the inserts firm. The inserts in such envelopes slide about, create an imbalance of the envelope, risking ripping and loss of contents. A snug fit keeps the enclosure firm in the envelope providing for effective mail handling.

Conversely, when an envelope is overstuffed it can burst at the seams when machine processed. The result can be a total loss of the mailing.

• The strength of the envelope should be such to withstand the weight of its contents. If there is stress at the seams or sharp edges, the envelope is in an overload condition where it can burst or tear apart and lose its contents.

  LETTER SIZE

Letter size envelopes must be rectangular in shape for automated processing by United States Postal Service.

The following dimensions must be met for automated processing by USPS:

	Minimum	Maximum
Height:	3 1/2 inches	6 1/8 inches
Length:	5 inches	11 1/2 inches
Thickness:	.007 inch	1/4 inch

  Nonstandard Size

Letter-size First Class Mail or single-piece -rate Standard Mail (A) weighing 1 ounce or less is nonstandard and subject to the applicable surcharge if is thickness exceeds .25 inches or if, based on the placement (orientation) of the address, its length exceeds 11.5 inches, its height exceeds 6.125 inches, or its length divided by its height - the aspect ratio - is less than 1.3 or more than 2.5.

Aspect Ratio

(not drawn to scale)

  FLAT SIZE

Flats are envelopes larger than 11 1/ 2 inches long by 6 1/8 inches high, but not larger than 15 inches long by 12 inches high.