How Do Postal Sizes Differ?

What are the common mailable sizes and how will my choice affect my postage rates?


When new to printing for mailings, sometimes there is confusion about the various sizes that post office allows for mailing and how it affects your postage rates. We have decided to create this breakdown to explain the most common postal sizes, letters and flats. Other sizes are allowed, but for most campaign mailings these sizes will most likely cover 99% of your mailings.

Letter Size Mail

What is considered letter size mail? 

According the the post office, a letter size mail piece is a mailer whose final size is between the following minimum and maximums:

Height:   Minimum - 3.5"   up to Maximum - 6.125"

Length:   Minimum - 5"   up to Maximum - 11.5" (*Folded Self Mailers have a maximum mailing length of 10.5")

Thickness:   Minimum - 0.007"   up to Maximum - -0.25"

What is the Aspect Ratio?

In order to get the automation discounts from the post office, letter size pieces must be rectangular in shape (wider than tall) and fall within a range of 1.3 to 2.5 when dividing length (dimension parallel to the address as read) by the height. So for example, a 6"x11" card has an aspect ratio of 1.833 and would be within the aspect ratio. However, a card measuring 5"x5.5" would have an aspect ratio of only 1.1, which is outside of the aspect ratio, which would cause the mailing to be subject to additional postage as non-automation surcharges.

What Can be mailed as a Letter?

Postcards, Letters Placed inside envelopes, or Folded Self Mailers. Sometimes new mailers don't realize that a jumbo postcard is considered a letter size by the USPS. The most common allowable sizes for postcards are 4"x6", 5.5"x8.5", 6"x9", and 6"x11". Folded self mailers can start out at any size, but in order to be mailed as a letter, the final size has to be between the minimum and maximum ranges. Enveloped mailers are most commonly a regular or window #10, or an invitation type such as an A6, although other mailable sizes are also available. Contact one of our Account Reps if you have questions about an allowable size.

How much postage does a letter size piece cost?

The discounts available for letters vary depending on the entry post office, the saturation of the mailing list and whether or not your piece meets the requirement for automation discounts. Most of the time if you are entering the mail into the local SCF Facility that serves the same area as your mailing list, you would qualify for the SCF rates. For example, a letter size mailing covering zip codes in San Jose, CA, if entered at the San Jose SCF Facility and automation compatible would cost about $0.22-$0.243 each. If you have a more highly saturated list, you could qualify for Enhanced Carrier Route rates which would bring your cost to about $0.158-$0.257 each. Non-SCF entry rates will likely run about $0.03 to $0.045 higher depending on the saturation.

SCF Entry Automation: $0.22-$0.243 each

None Entry Automation: $0.251-$0.287 each


SCF Entry Enhanced Carrier Route: $0.158-$0.257 each

None Entry Enhanced Carrier Route: $0.186-$0.292 each

So, What is a Flat Size Mailer

What is considered flat size mail? 

When any of your dimensions exceed any of the maximums for letter sizes it changes the mailer from a letter to a flat size. So an 11"x17" piece that folds to an 8.5"x11" would be considered a flat size because the final height dimension would exceed the 6.125" maximum for letters. Interesting thing about flats is that the longest dimension is considered the length.


Height:   Minimum - over 6.125"   up to Maximum - 12"

Length:   Minimum - over 11.5"   up to Maximum - 15" (Folded self mailers exceeding 10.5" in length are considered flats)

Thickness:   If thickness is over  - 0.25"   up to  Maximum - 0.75"


Does the Aspect Ratio apply to flat sizes?

No. You could mail a flat that is 8"x8" without worrying about the aspect ratio. In this case one dimension exceeds the maximum for letters so it is a flat size.

How much postage does a flat size piece cost?

The discounts available for flats also depends on the entry post office, the saturation of the mailing list and whether or not your piece meets the requirement for automation discounts. Assuming the same zip codes and entry as the letter example above, the SCF postage rates would range from $0.34 to $0.514 per piece. In the case where you have a highly saturated list, the rates range from $0.165 - 0.257 per piece, which can be very close to the letter size rates.

SCF Entry Automation: $0.34-$0.514 each

None Entry Automation: $0.393-$0.598 each


SCF Entry Enhanced Carrier Route: $0.165-$0.257 each

None Entry Enhanced Carrier Route: $0.219-$0.295 each

Advantages of Letter Size vs. a Flat size?

Letter size:

  • Great option when you want to spend less on printing.
  • Works well when your printed message can easily fit inside a standard envelope size.
  • Postage is reasonable compared to a flat size for less saturated mailing lists that are mostly automation sorted.

Example: Mailing a fundraising appeal letter in a #10 envelope to voters of one party across several zip codes.

Flat size:

  • More noticeable in the customer's mail box because of larger size
  • Postage can be nearly the same as a letter size if the list is highly saturated, CRRT sorted.
  • Tabbing/Sealing  is usually not required for Flats, saving you mail production cost

Example: Mailing a 8.5"x11" jumbo postcard or flyer to all addresses within a single zip code.


Hopefully this helps you understand the difference between a letter and a flat. If you have questions about other sizes, contact one of our account reps using the Contact Us link at the top of the page and we will be happy to help.