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USPS Size Categories & Mailing Information

This article will discuss how mail piece size and design can relate to your campaign goals.

A postcard is just smaller single sheet mailed flat and not in an envelope? Isn't a letter a sheet of paper folded and stuffed in an envelopes? All things are Flat when mailed? .......All true in most cases

However the Postal Service uses specific terms to categorize mail pieces by their size. Each of those sizes corresponds to a rate per piece. These terms "Postcard, Letter and Flat" have no relationship to what you might consider a "letter" to be or for a "flat" - no correlation to a shape or position.

We will not include any information relating to parcels or packages as we do not offer those services online. Thus the products we offer are represented in (3) main categories/rates by the Postal Service.

First we have to consider where the Address Block (the recipient name, address, city, State, Zip and barcode) are positioned or printed with regard to the bottom of the mail piece and with respect to the leading edge. Then we can determine the size and thus the Postal Service Category.

For example, both of the cards below measure 8.5" x 5.5" but to USPS where the address block is represented by the pink shaded boxes. The design or  placement can determines the category as well as the size.  The address blocks orientation with the bottom of the mail piece thus determine the 'width" in most cases.


The Letter above is 8.5" wide and 5.5" tall The Flat is 5.5" wide and 8.5" tall.

Postcard Rate

A postcard is a single piece of paper or cardstock that is within a narrow size range 5" to 6" wide/width and 3 1/2" to no more than 4 1/4" tall/height.

Postcards Details:
Rectangular in shape; and,
At least 3½ inches tall but no more than 4¼ inches tall; and,
At least 5 inches wide but no more than 6 inches wide; and,
At least 0.007 inches thick but no more than 0.016 inches thick
Are not mailed in envelopes.

These can be a perfect fit for a "Save the Date" or seminar or if you are considering First Class Postage and have a quantity over 500, these will move fast in arrive quickly in home in most cases. You are limited on the back side as much of the area is needed for the address and to get the best rates, we need to ensure the mailing area is readable. However there still is room for your content or messaging.  See one of our regular postcard templates to get a better idea or download our design guidelines

Letter Rate:

Most of our business in marketing and promoting for agents, uses the larger postcards sizes  - Jumbo 5.5" x 8.5" or Super Jumbo 6" x 9" or our Tall Panoramic 6" x 11". For these sizes since they are similar to the above postcard details, but is larger than 4¼ inches tall and 6 inches wide, then technically in the USPS world these are not in the  "postcard" category but in the "Letter" category.

A letter is anything that starts out at the minimum size for postcards (see above) and goes up to a maximum of 6⅛ inches tall and 11½ inches wide.

Letter Details:
Rectangular in shape; and,
At least 4 1/4" inches tall but no more than 6 1/8" inches tall; and,
A maximum of 11 1/2" inches wide and
At least 0.007 inches thick but no more than 0.016 inches thick
Are not mailed in envelopes.

Anything larger than the maximum size for a letter is categorized as a "Flat" the next size we will discuss below.


This goes all the way up to a max size of 12 inches tall and 15 inches wide. Anything in between the max size of a letter and this 15" x 12" size is considered a flat. It is also important to remember that as we saw above regardless of size if your address block is placed in certain orientations or locations, even a letter can be a flat.

Flats can be folded as well for example we offer a 23" x15 Double Fold Mailer. The layout for these can be complicated and will vary form printer to printer however it is important that the address block, fold location and leading and trailing edges all meet the USPS requirements, so just reach out to us or use one of our templates. The impact theses can make however can be incredible!

Flats, have considerable higher postage costs than postcards and letters. For smaller quantities (200-400) with low saturation, such as SOI or client lists, the cost impact is greatest for Presort Class and First Class quantitates under 500 as well will incur a significant incremental cost when compared to postcards and letters.  More saturated farming lists may be more manageable. Additionally, delivery is often slower for this size, with even first-class mail taking 3-10 days to arrive. Keep this in mind when planning your campaign or event. To expedite delivery, we sometimes add extra sorting and transport to another facility.

This illustration that might help.



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