USPS Classifications Of Mail

Letters or Flats?

USPS terms for defining classifications of mail can be confusing.  These classifications are based on the size and shape of the mail piece; not only do they determine postage, but they are routed across the country in a different manner as well.

When you are preparing your mail piece, it is important to know what classification your mail fits into since this will directly impact your postage budget.

Letters – machinable and nonmachinable

Flats – machinable and nonmachinable

Machinable parcels

Irregular parcels

Outside parcels



  • Height is at least 3 ½” and no more than 6 1/8”
  • Width is at least 5” and no more than 11 ½”
  • Thickness is at least 0.007 and no more than ¼”

Letters that can be sorted on automated equipment are eligible for better postage prices than those that are nonmachinable.

Machinable letters meet all of the criteria above and

  • Weigh no more than 3.3 ounces
  • Are rectangular in shape – it should meet the aspect ratio, which can be determined by dividing the length by the height.  Aspect ratio should be not less than 1.3 or more than 2.5.

Examples of nonmachinable letters include

  • Weight of more than 3.3 ounces
  • non-rectangular
  • enclosed in any type of plastic material
  • includes strings/buttons/clasps
  • contains items such as pens, coins or keys which cause the thickness of the mail piece to be uneven
  • too rigid to meet flexibility requirements
  • has the address placed the same direction as the shortest side of the mail piece instead of the longest side,
  • is a selfmailer with the folded edge perpendicular to the address
  • is a booklet that does not meet USPS

Barcoded mail pieces are eligible for discounted postage rates and must meet additional requirements beyond those for machinable letters.  Always check with your mail vendor for eligibility.


Special postage rates are given only for First Class postcards meeting eligibility.  There is no postcard rate for Standard Class.

Eligibility for First Class postcard price include:

  • Rectangular in shape (see how to determine aspect ratio above)
  • At least 3 ½” high and not more than 4 ¼” high
  • At least 5” long and no more than 6” long
  • Thickness is at least 0.007” and no more than 0.016”

Cards larger than outlined above should use Standard Class letter or flat pricing, depending on the size of the card.

This covers the very basics of letters and postcards.  Next week we will address the confusing world of flats!

As always, if you have questions or want additional information, our Customer Service Representatives are here to serve you.

  1. #1 by Paula Graves Adams on October 31, 2011 - 9:38 pm

    “Are rectangular in shape – it should meet the aspect ratio, which can be determined by dividing the length by the height. Aspect ratio should be not less than 1.3 or more than 2.5”

    Assume this means that vertical postcards won’t fly anymore? That is, stamp on upper right of short end?

    Do address label and stamp both need to read in same direction? That is, if design of back (address side) of a postcard is vertical, but (assuming above true) stamp is in upper right horizontally, does address label have to also read horizontally (lengthwise) to postcard?

    This is great. So appreciate. Have been in USPS guidelines but I never get this far.

  2. #2 by Agustin Hooe on November 1, 2011 - 5:16 pm

    You should take part in a contest for one of the best blogs on the web. I will recommend this website!

    • #3 by Kimberly Tarpley on November 7, 2011 - 10:17 am

      Thank you for the kind feedback, Agustin! I appreicate you reading our little blog.

      Kimberly Tarpley
      Strahm Automation

  3. #4 by Teodoro Thurner on November 2, 2011 - 6:54 pm

    Would you be involved in exchanging hyperlinks?

  4. #5 by Kimberly Tarpley on November 4, 2011 - 8:41 am

    Hi Paula;

    Great questions!

    You can still mail vertical postcards. However, this renders them non-machinable and will result in a higher postage rate.

    The address and postage should read in the same direction. Of course, if you put enough postage on it, you can mail virtually anything (with a few exceptions).

    You can find more information about postcards here:

    Please let me know if there are any other questions I can answer for you!


    Kimberly Tarpley
    Strahm Automation

  5. #6 by Rayford Teekasingh on November 7, 2011 - 6:15 am

    It?s hard to find knowledgeable people on this subject, however you sound like you understand what you?re speaking about! Thanks

    • #7 by Kimberly Tarpley on November 7, 2011 - 10:19 am

      Hi Rayford;

      Thanks for the comment. It can be awfully tough for people to keep up with, let alone understand, all the rapidly changing USPS rules and regulations. I do my best to keep our customers informed!

      I hope you keep reading our blog.

      Kimberly Tarpley
      Strahm Automation

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