USPS Classifications Of Mail – Part 2

Is it a Letter or a Flat – Part 2

Last week we delved into the basics of letters and postcards.  This week we will review the abundance of information regarding flats and parcels.  It’s a lot, so hold on to your hats!


A flat is described as a rectangular or square piece of mail that is too big in at least one direction to be a letter.  That is, larger than 6 1/8” or 11 ½” or thicker than ¼”.

Just to confuse matters more, the mail piece cannot have a side longer than 15”, a side shorter than 12”, is not thicker than ¾” and does not weigh more than 13 ounces for First Class mail or 16 ounces for Standard Class.

USPS recently changed the rules for flats and all pieces must now meet certain flexibility standards. There is a complicated test to determine flexibility; it is always wise (and suggested!) that you discuss this with either your local Postmaster, Business Mail Analyst or your mail presort vendor.

Significantly higher postage rates are charged for mail pieces that don’t meet these flexibility requirements.  This would include items such as CD, DVD or boxes.  Pieces too rigid to qualify as flats are generally categorized as parcels.

Periodicals get their own special class and must meet requirements to fit one of two sorting processes.  They, too, fall into either machinable or nonmachinable catagories.

Standard Class has a category not flat-machinable (NFM). This is a piece that would normally be a flat, with the exception that it does not meet flexibility requirements. This includes boxes, CD/DVD or anything with a non uniform thickness.

Not flat-machinable mail pieces have a higher postage rate because they must be sorted by hand.


As with other mail classes, there are two groups of parcels – machinable and nonmachinable.

A machinable parcel is regular in shape (not including tubes, rolls, etc) and is able to be processed on automated sorting equipment. These mail pieces are also too large or rigid to be considered a flat.

Must meet all of the following:

  • Must be at least 6” but not more than 34” AND
  • Height must be at least 3” but not more than 17” AND
  • Thickness must be at least ¼” but no more than 17” AND
  • Must weigh at least 6 ounces but not more than 35 pounds (except books and printed matter, which cannot be more than 25 pounds).

Irregular parcels may includes most everything that cannot be called a letter, flat, NFM or machinable parcel.  This includes rolls, tubes and anything else that is too big or too irregular to be sorted on
automated equipment.

An Outside parcel exceeds the size limitations of machinable parcels. This includes tubes or rolls longer than 26”, strapped boxes, metal or wood boxes, cartons containing 1 or more gallons of liquid and other out of the ordinary pieces.

It bears repeating, your best action is always checking with your local Postmaster, Mail Acceptance Analyst or presort mail vendor.   

Strahm Customer Service Representatives are available and knowledgeable  in answering all your mail piece questions!

  1. #1 by Tashia Busl on November 4, 2011 - 2:55 pm

    Keep working ,terrific job!

  2. #2 by Kimberly Tarpley on November 7, 2011 - 10:15 am

    Thank you, Tashia!

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