Physical Standards

Mail Piece Dimensions

Since machinable mail is processed by automated equipment that moves at high speeds, it must fit within either Letter Mail or Flat Mail dimension guidelines. You need to consider the final size of your mailing, since separate postage rates are set for each size range.

The cost of mailing varies with each classification.
Dimensional standards for cards
Dimensional standards for letters
Dimensional standards for flats


The United States Postal Service has different rates for different sized publications.
Postcards must be at least 3-½" x 5" and a maximum of 4-¼" x 6" to qualify for the postcard rate ($0.28). Postcards that exceed the maximum size are charged letter-rate postage ($0.44 for first class mail). Postcards that are mailed Non-Profit Bulk Rate are currently charged at the letter-size ($0.17) if they do not exceed the maximum size (6⅛ x 11½"). An important note for oversized postcards – they must be printed on a stock that has a minimum thickness of .009” (9 point). This does, unfortunately, limit you to white colored stock.

Letter-Size mail cannot exceed 6-⅛" x 11-½". Larger pieces are considered Flat-Size and the postage rates are higher. This means departmental newsletters, for example, developed with page sizes of 8-½" x 11" will be mailed out at the flat rate unless you want to have them folded down to 5-½" x 8-½" and tabbed to be able to mail them at the letter rate.

Generally speaking, folding a document to a letter-sized dimension is going to save on postage. Typically, newsletters have a page size of 8-½" x 11". If mailed out at this size, the postage rate would be for the Flat-size, and cost a maximum of $0.42 each (non-profit). The same document, folded to 5-½" x 8-½" could be mailed at the letter-sized rate of $0.17 each. This would save the customer up to $0.25 in postage on each piece mailed. In most cases, the extra cost of folding is more than made up by the savings in postage.

The exception is when the document contains more than 12 pages, or if it is to be saddle-stitched (two staples in the middle along the fold…like a magazine). Saddle-stitched publications must be folded by hand after they have been bound, and usually, the additional cost of the folding, and the need for tabbing the piece to hold the fold, will cost more than can be made up by the savings in postage. Customers with newsletters and other publications that exceed 12 pages or are saddle-stitched are encouraged to mail the documents at the Flat-Size Rate.

PLEASE NOTE: When producing self mailing letter-size pieces, such as brochures, paper choice is important. Your piece will need to be produced on 60# paper if it folds in thirds or heavier it folds in half to be mailable. If it is produced on 20# paper it will not met USPS codes for mailing. To save time and money always check first.

Dimensional Standards for Cards –

Each card claimed at a card rate must be:

  1. Not less than 3-½" high, 5" long, and 0.007" thick.
  2. Not more than 4-¼" high, or more than 6" long, or greater than 0.016 inch thick.
  3. Cards over this size (4-¼” x 6") will go at letter rate.

•  Address Side of a Card
The address side of a card is the side bearing the delivery address and postage.
The address side may be formatted to contain a message area. Cards that do not contain a message area on the address side are subject to the applicable standards for the rate claimed. Miscellaneous graphics or printing, such as symbols, logos, or characters, that appear on the address side of cards not containing a message are generally acceptable. Cards claimed at the Presorted or automation card rates that contain a message area on the address side must be divided according to the following standards:

  • The address side of the card must be divided into a right portion and a left portion, with or without a vertical rule. The left portion is the message area.
  • The delivery address, postage, and any permit marking or endorsement must appear in the right portion. The right portion must be at least 4" wide (measured from the right edge of the card, top to bottom inclusive).
  • A minimum area of 4-½" x ½" across the bottom must be left blank for the barcoding.

Dimensional Standards for Letters –

Letter-size mail is:

  1. Not less than 5" long, and 3-½" high, and 0.007" thick.
  2. Not more than 11-½" long, or more than 6-⅛" high, or greater than ¼" thick.
  3. Not more than 3.5 ounces (First-Class Mail letter-size pieces over 3.5 ounces pay flat-size rates).

Letter Mail Dimensions

Letter Mail primarily consists of business envelopes, flyers, and folded self-mailers that fit within the dimensions listed above. It is also necessary for the mail pieces to be horizontally rectangular. If not, they tend to tumble and jam during transport through the processing equipment.

A nonmachinable surcharge can apply to some Standard Mail letter-size pieces. Pieces that are nonmachinable are excluded from automated processing and must be handled manually. Nonmachinable pieces also may impede mail flow or damage the mail or mail processing equipment. Manual letters are considerably more costly to process than machinable letters.

Dimensional Standards for Flat-Size Mail –

Flat-size mail is:

  1. More than 11-½" long, or more than 6-1/8" high, or more than ¼" thick.
  2. Not more than 15" long, or more than 12" high, or more than ¾" thick.
  3. Flexible.
  4. Rectangular with four square corners or with finished corners that do not exceed a radius of 0.125" (⅛").
  5. Uniformly thick.

Standard Flat-mail size diagram• Length and Height of Flats
The length of a flat-size mailpiece is the same as the printed address direction. The height is the dimension perpendicular to the address.