Postal Regulations on Sizes of Envelopes

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••• corner of envelopes image by timur1970 from

The size of an envelope dictates the type of mailing services available when shipping through the United States Postal Service (USPS). The post office regulates the size of envelopes for the cheapest mailing option, first class mail. In addition to specifying minimum and maximum length and width, variations in depth are also accessed to determine if a mail piece is machinable, or able to move through automated sorting machinery at distribution centers.

First Class Mail Letters

USPS first class mail features both minimum and maximum size parameters for envelopes. If a letter or bubble mailer exceeds the limits, it moves up to a more expensive shipping option, while smaller letters are not allowed due to sorting issues.

Dimension Ranges:

  • Width: 3 1/2 inches to 6 1/8 inches
  • Length: 5 inches to 11 1/2 inches
  • Depth: 7/10000 inch – the approximate thickness of an index card — to 1/4 inch

First Class Mail Large Envelopes

A subset of first class mail known as first class flats allows for the shipment of large envelopes, such as manilla mailers and thinner, padded envelopes. Flats ship at a higher rate than a standard letter but for cheaper than a larger envelope. Larger envelopes can ship as a first class parcel or in a different shipping category.

Dimension Ranges:

  • Width: 6 1/8 inches to 12 inches
  • Length: 11 1/2 inches to 15 inches
  • Depth: 1/4 inch to 3/4 inch 

Nonmachinable Surcharges

Even if your envelope falls within the measurement requirements dictated by the USPS, you can still face additional charges if the letter or mailer features nonmachinable characteristics. The postal service uses a variety of automated machinery to sort letters and prepare them for delivery. When a letter is rigid, uneven or square, it cannot move safely through the machinery and requires additional processing, so an additional fee of $0.21 is charged.

How to Reduce Nonmachinable Surcharges:

  • Remove clasps from manilla envelope mailers. If the envelope is not self-adhesive, use tape to close. 
  • Avoid adding messages, such as "Do Not Bend," to an envelope. If it is imperative that an item is not bent, prepare to pay the extra postage. 
  • Do not add cardboard for support. This creates a rigid mailer. 
  • Place a square card inside a rectangular envelope to protect it during shipping. 

Additional Mailing Options

The post office provides free envelope mailers for more expensive mailing options, such as Priority Mail and Express Mail. Bubble mailers, cardboard envelopes and fiber envelopes of various sizes can be requested through the USPS website and used to mail products of various sizes, weights and depths. Because Priority Mail, Express Mail and retail ground shipping are package services, no size parameters exist specifically for envelopes. Instead, package size is limited to a maximum length plus girth – the distance around the box at its largest point – of 130 inches and 70 pounds.


  • A standard letter envelope must be no smaller than 3 1/2 inches by 4 inches and no larger than 6 1/8 inches by 11 1/2 inches, according to postal regulations. Larger sizes are allowed in different first class mail categories.

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