USPS Regulations & Specs for Letters & Postcards

Postcard Dimensions
postcard_small Minimum Maximum
Length 5″ 6″
Height 3-1/2″ 4-1/4″
Thickness 0.007″ 0.016″
Letter Dimensions
letter_small Minimum Maximum
Length 5″ 11-1/2″
Height 3-1/2″ 6-1/8″
Thickness 0.007″ 1/4″

Mailpieces are classified by shape and by the way they are prepared. These classifications are based on how efficiently mailpieces can be processed on Postal Service equipment. The Postal Service classifies letters and cards into one of three categories: machinable, nonmachinable, and automation.

Machinable Letters and Cards:

If you prepare your letters and cards so that they have an accurate address and can be processed on Postal Service equipment, your mail is “machinable” and eligible for “presort” prices. Machinable mailpieces must meet specific standards, including size, shape, and weight.

Maximum weight: First-Class Mail, machinable letters and Standard Mail machinable letters: 3.3 ounces, Periodicals letters: 3.5 ounces.

Machinable letters must not meet any of the nonmachinable characteristics.


  • Minimum: 5 inches long, 3-1/2 inches high, and 0.007 inch thick.
  • Maximum for First-Class Mail card prices: 6 inches long, 4-1/4 inches high, and 0.016 inch thick.
  • Maximum for letters and other cards: 11-1/2 inches long, 6-1/8 inches high, and 1/4 inch thick.
  • Rectangular, with four square corners and parallel opposite sides. Letter-size, card-type mailpieces made of cardstock may have finished corners that do not exceed a radius of 1/8 inch.

Nonmachinable Letters:

Maximum weight: First-Class Mail and Periodicals nonmachinable letters 3.5 ounces; Standard Mail nonmachinable letters; less than 16 ounces.

  • Presorted First-Class Mail letters are subject to a nonmachinable surcharge if they meet any of the characteristics. The nonmachinable characteristics do not apply to pieces mailed at card prices.
  • Standard Mail letters that have one or more of the nonmachinable characteristics are subject to the nonmachinable letter prices.

Examples of a nonmachinable letter include:

  • It has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5 (a square envelope has an aspect ratio of 1, making it nonmachinable).
  • It is more than 4-1/4 inches high or more than 6 inches long and is less than 0.009 inch thick.
  • It has clasps, strings, buttons, or similar closure devices.
  • It is too rigid.
  • It has a delivery address parallel to the shorter side of the mailpiece.
  • It contains items such as pens, pencils, or keys that create an uneven thickness.
  • Letters with nonpaper surfaces, other than envelope windows or attachments that are allowed under eligibility standards by class of mail.

Automation Letters and Cards:

If your letter-size mailpiece is machinable and displays the correct Intelligent Mail barcode, you may qualify for lower, “automation” prices. A letter or card meets automation standards and qualifies for automation prices if it meets the specific addressing, barcoding, and design standards.

Maximum weight:

  • First-Class Mail automation letters: 3.5 ounces.
  • Periodicals automation letters: 3.5 ounces.
  • Standard Mail automation letters and carrier route letters: 3.5 ounces.

Shape: rectangular. Aspect ratio (length divided by height): 1.3 to 2.5


  • Minimum: 3-1/2 inches high, 5 inches long, and either 0.007 inch thick if not 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.
  • Maximum for First-Class Mail card prices: 4-1/4 inches high, 6 inches long, and 0.016 inch thick.
  • Maximum for letters and other cards: 6-1/8 inches high, 11-1/2 inches long, and 1/4 inch thick.

Length is the dimension parallel to the address as read.

For Designing Flats for Automated Processing refer to the USPS Quick Service Guide 201d, Commercial Flats, Designing Flats for Automated Processing.

For Physical Standards for Commercial Flats refer to the USPS Quick Service Guide 201c, Commercial Flats, Physical Standards for Commercial Flats.