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Letters, Flats and Parcels Explained

All trees are plants, but not all plants are trees. There is a similar connection when it comes to explaining what the post office refers to as a letter, a flat or a parcel. One may logically think that a letter refers to a single sheet of paper, perhaps inserted into a standard business envelope for mailing. Although this could qualify as a letter to the post office, this is not the definition of a letter in postal terms.

To the post office, the terms letter, flat and parcel refer more to categories that are based on a range of physical attributes rather specific types of mailpieces. There are other categories and sub-categories such as customized market mail, periodicals, bound printed matter, etc., but for the purposes of this article I will just go through the basic three. The post office defines a machinable letter as any mailpiece that complies with the following attributes:

  • Height between 3.5 inches and 6.125 inches
  • Length between 5 inches and 11.5 inches
  • Aspect ratio (length divided by height) between 1.3 and 2.5
  • Thickness between 0.007 inch and 0.25 inch (at least 0.009 inch thick if more than 4.25 inches high or 6 inches long or both)
  • Weight no more than 3.5 ounces
  • Rectangular in shape, having four right-angle corners with no more than a 0.125 inch radius and parallel opposite sides

Note that these specifications are for machinable mailpieces. Any attribute that does not comply with these eligibility standards may hinder the mailpiece non-machinable and would result in a higher postage rate.

The post office defines a machinable flat as any mailpiece that complies with at least one of the following attributes:

    • Height more than 6.125 inches but no more than 12 inches
    • Length more than 11.5 inches but no more than 15 inches
    • Thickness more than 0.25 inch but no more than 0.75 inch
    • Weight more than 3.5 ounces but no more than 16 ounces

There is no aspect ratio specified for flats. However, they do need to be rectangular, which is defined as having four right-angle corners with no more than a 0.125 radius and parallel opposite sides. A square (four sides of equal length) is considered a machinable flat.

A parcel gets a bit more complex. There are several categories of parcels including machinable, irregular, non-machinable, lightweight, Parcel Select, bound printed matter and marketing parcels to name a few. I will focus on a basic machinable parcel for comparison purposes. The post office defines a machinable parcel as any mailpiece that complies (in addition to the attributes of a flat) with at least one of the following attributes:

    • Height more than 12 inches but no more than 17 inches
    • Length more than 15 inches but no more than 27 inches
    • Thickness more than 0.75 inch but no more than 17 inches
    • Weight more than 16 ounces but no more than 25 pounds

In addition to the basic physical attributes in each category, there are many other factors such as type of material, poly wrapping, flexibility and uniform thickness that can effect whether a mailpiece is deemed machinable or not. Letters, flats and parcels are processed through different types of equipment and therefore can have different physical requirements. Flexibility and uniform thickness are two of the more common aspects that can affect postage rates. A flat must comply with a flexibility and deflection tests to ensure the piece has the proper flexibility (not too rigid and not too flimsy) to move through postal equipment without issue. This is determined by positioning the mailpiece so that it hangs off of a ledge and measuring how much the piece droops or can be bent. A machinable flat must also be uniform in thickness, meaning any variation in thickness, excluding 1 inch from the edge, must not be more than 0.25 inch.

Generally, if any aspect of a mailpiece does not comply with the eligibility standards in a category then it would incur the postage prices for the next category up. For example, consider a basic postcard that is 6.5 inches by 10 inches. The dimension of 6.5 inches in height (as this is the shortest of the two dimensions) is outside the eligibility of a letter. Therefore, the mailpiece would be deemed a flat and incur postage prices for a flat. If the postcard were 10 inches by 16 inches it would be deemed a parcel since 16 inches is outside either maximum dimension for a flat.

It may seem confusing to think of a postcard as a letter, flat or parcel, but common and seemingly similar direct mail pieces can fall into a variety of pricing categories for a variety of reasons. When a mail house refers to your EDDM postcard as a flat that is likely because of the dimensions required for EDDM simplified addressing. When the mail house refers to your large envelope as a parcel that may be because of the multiple books inserted in it are causing a non-uniform thickness or flexibility issues.

There are many more regulations and specifications listed in postal service’s Domestic Mail Manual. The DMM specifies everything about all things mail and is not for the light reader. An online version of the DMM along with other resources such as quick service guides and pricing sheets can be found on the postal service’s Postal Explorer website: Another fairly new resource is their Postal Pro search engine: This website took the place of the RIBBS website and is where you can find various documents related to retail and commercial mail. The full listing of physical standards for commercial mail can be found here:

If you have any questions regarding designing a mailpiece for commercial mail or planning a direct mail campaign, feel free to call Phillips Printing mailing department at 888-ask-phil or contact us.