accurate mailing service

USPS Regulations –
Standard Sizes for Letters, Postcards & Flats

USPS regulations are incredibly important to understand for mailing anything in the United States. The USPS has specific rules set out for standard postcard size & other standards for letters or flats for a smooth automated flow through the post office. Things like barcodes, stamps, indicias, mailing addresses, etc. are needed to allow your mail to end up at the intended recipient. Postal regulations can get complicated and can even change from month to month or day to day but we can help you understand them here or you can contact us.

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Size Standards

Standard Sizes for Letters

Letters are a common and affordable way to present information to a client. Many of our clients’ mail in the upwards of thousands of letters. Here are the important things you need to know when you create your mail piece:

  • The letter must be no less than 5 inches long, 3½ or 3.5 inches high, and 7/1000 or 0.007 – inch thick.
  • Mail pieces that are more than 4¼ or 4.25in high or 6 in long, or both, the minimum thickness is 9/1000 or 0.009in.
  • No more than 11 ½ or 11.5in long, or more than 6 ⅛ or 6.125in high, or more than 0.25in thick.
  • No more than 3 ½ or 3.5oz (First-Class Mail letter-size pieces over 3.5oz pay flat-size prices).
  • Rectangular, with four square corners and parallel opposite sides. Letter-size, card-type mailpieces made of cardstock may have finished corners that do not exceed a radius of 0.125 or ⅛ in.

Standard Sizes for Postcards

Postcards are an affordable method of getting your message out without the hassle of envelopes or folding. Dimensional standards for postcards are:

  • No less than 5 inches long, 3½ or 3.5 inches high, a minimum thickness of 7/1000 or 0.007 – inch thick, no more than 4¼ or 4.25in high or 6 in long, or both.
  • No more than 6in long, or more than 4 ¼ in or 4.25 in high, or more than 0.016in thick.
  • Rectangular, with four square corners and parallel opposite sides. Card-type mailpieces made of cardstock may have finished corners that do not exceed a radius of 0.125 or ⅛ in. You will pay a larger amount for di-cut style postcards.

Side note: from our experience, for a school bus die-cut postcard one of our clients ended up paying upwards of $30,000, just in postal fees. That is because these types of cards cannot run through the automated machines at the post office.

Standard Sizes for Flats

  • Flats are what the Postal Service refers to large envelopes, newsletters, and magazines.
  • Must be over 11½ or 11.5in long, or more than 6⅛ or 6.125in high, or more than ¼ or 0.25in thick, except as allowed for EDDM.
  • For general retail mailability, all pieces ¼in thick or less must be a minimum of 5in long and 3.5 or 3½in high and 7/1000 or 0.007in thick.
  • No more than 15in long, or more than 12in high, or more than 0.75 or ¾in thick.
  • Flats must be flexible.
  • Rectangular with four square corners or with finished corners that do not exceed a radius of 0.125 or ⅛in.
  • Uniformly thick so that any bumps, protrusions, or other irregularities do not cause more than 0.25 or ¼in variance in thickness.
  • Unwrapped, sleeved, wrapped, or enveloped.

Postal Elements

Endorsement

The mailer is responsible for the correct endorsement and placement of the endorsement on each mailpiece to provide delivery instruction or to request an ancillary service (forwarding, return, or address correction), subject to the corresponding standards for use and availability. Placement of endorsement on the mailpiece is determined as follows:

  • A retention period specified by the mailer must be placed directly above the return address.
  • Any ancillary service endorsement (e.g., Address Service Requested, Forwarding Service Requested, Return Service Requested, Change Service Requested) must be placed in one of these four positions:
    • Directly below the return address.
    • Directly above the delivery address area (which includes the delivery address block and any related non-address elements such as a barcode, keyline, or optional endorsement line).
    • Directly to the left of the postage area and below or to the left of any price marking.
    • Directly below the postage area and below any price marking.
  • The carrier release endorsement “CARRIER – LEAVE IF NO RESPONSE” must appear directly to the left of the postage area (preferred) or be placed directly below the return address.
  • A minimum ¼ inch clear space must appear between any other printing and the carrier release endorsement.
  • If an ancillary service endorsement is used, the carrier release endorsement must be separated from the ancillary service endorsement by the equivalent of one blank line of the type size used.

Return Address
When a printed ancillary service endorsement is used, or a request is embedded within an Intelligent Mail barcode, a domestic return address must be placed in the upper left corner of the address side of the mailpiece or the upper left corner of the addressing area. If the return address is multiple delivery addresses, it must show a unit designator (apartment number or something of the sort).

Class of Mail
There are multiple different mailing classes to choose from when you are mailing. These classes are determined by things like whether you are a non-profit or not, how many pieces you want to mail, and how much you are willing to pay to get it out quickly. You can choose from:

  • Priority Mail Express
  • Priority Mail
  • First-Class
  • USPS Marketing Mail
  • Non-profit (must be an established non-profit organization with mailing permit)

Each class has its own benefit so it is wise to find out which one will be the best for your mailing.

Postal regulations are a very complex topic and unfortunately, the USPS changes the regulations frequently. We are here to help with any questions and can even guide you through the mailing process. Some information within this article may be subject to change. We have tried to compile the most important pieces of information for you to have a successful mail drop. Things like barcodes, stamps, indicias, dimensional standards, etc. are important to allow for a smooth flowing process through the post office and hopefully, our information will give you good insight on how to correctly set up a mail piece.