USPS Classifications of Mail

Letters or Flats?

letterpcUSPS 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.


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