Learn About Bulk Mailing and Tracking → Postcard, Letter, Flat or Parcel

Is it a Postcard, a Letter, a Flat or a Parcel?

 Scott
 Postage $aver Software


These are the four main size categories that USPS uses to classify mail and determine postage. Is your mail a postcard, letter, flat or parcel? Here's the quick answer:

A postcard is a card (not an envelope) that fits ALL THREE of the following:

• 3½ - 4¼ inches tall; and,
  
• 5 - 6 inches wide; and,
  
• 0.007 - 0.016 inches thick.
  
  For Marketing Mail (bulk mail), the postcard category does not apply. Postcard-size pieces are classified as letters.
  
  For Presorted First Class mailings only, the maximum postcard size is 6" x 9".

A letter is a rectangular card or envelope that fits ALL THREE of the following:

• 3½ - 6⅛ inches tall; and,
  
• 5 - 11½ inches wide; and,
  
• 0.007 - ¼ inches thick
  (except at least 0.009" thick if the piece is larger than 4¼" x 6").
  
• weighs no more than 3½ ounces (applies to First Class letters only).
  
  If your letter is an unusual size, shape or material, or have unusual content, find out if you have to pay a surcharge. For First Class mail only (including regular mail and Presorted First Class), a card that is postcard-sized is a postcard, not a letter.

A flat is a rectangular card or envelope that is larger or heavier than a letter, meeting AT LEAST ONE of the following:

• more than 6⅛ inches on its shorter side; or,
  
• more than 11½* inches on its longer side; or,
  
• more than ¼ inches thick.
  * 10½ inches for Every Door Direct Mail.

AND, meeting ALL of the following:

• no more than 12 inches on its shorter side; and,
• no more than 15 inches on its longer side; and,
• no more than ¾** inches thick.
  ** 1¼ inches for Periodicals class nonmachinable pieces.

To be classified as a flat, a mail piece must also:

• be flexible enough to go through postal sorting equipment. For example, a carton, hard-cover book, or an envelope with contents that cannot bend is a "parcel" even if it fits the dimensions for a flat.
  
• not be "lumpy". The thickness may not vary more than ¼" except within 1" of the edges.

A parcel is an envelope, carton, tube, or anything else that fits ANY one of the following:

• longer than 12 inches on its shorter side; or,
  
• longer than 15 inches on its longer side; or,
  
• more than ¾ inches thick (except more than 1¼ thick for Periodicals class nonmachinable pieces); or,
  
• fits the dimensions for a flat, but is not flexible, or is too lumpy.
  

Read on if you need more details on postcards, letters, flats and parcels.

If you are preparing bulk mail, the very first thing you have to know is what mail size category your mail piece fits into. There are four basic size categories: postcards, letters, flats and parcels.

There are different postage rates for mail in each size category. For bulk mail, the size category also determines how you will sort the mailing, since different mail sorting centers handle different types of mail.

Unfortunately, the terms used for each of these categories are confusing. For example, sometimes when you mail a "letter", the postal service classifies it as a "flat", while sometimes when you mail a flat card, the postal service classifies is as a "letter".

The basic rule to remember is that these categories are based on the size of the piece, not the content.

Our Postage $aver bulk mail preparation software automatically determines the correct sorting and postage for the category of mail you are sending.

Here's what the rest of this page covers:

What are postcards?

What are letters?

What makes a letter machinable vs. nonmachinable?

What are flats?

What are parcels (including machinable and nonstandard parcels)?

What are marketing parcels?

While most people consider any card that goes through the mail to be a "postcard", that's not the USPS definition. The only cards that are actually classified by USPS as "postcards" are cards (not envelopes) that:

• are rectangular in shape; and,
• are at least 3½ inches tall but no more than 4¼ inches tall; and,
• are at least 5 inches wide but no more than 6 inches wide; and,
• are at least 0.007 inches thick but no more than 0.016 inches thick.

For Presorted First Class only, the maximum postcard size is 6" x 9".

Postcards get special pricing if you are mailing them at First Class retail (regular mail) rates or at Presorted First Class rates. First Class retail postage for a postcard is just 56¢, compared with 73¢ for a letter, with similar discounts in Presorted First Class postcard rates.

But for every mail category other than First Class retail and Presorted First Class, postcards are classified as "letters", and don't get any special postcard discount.

That means that if you are sending Marketing Mail bulk mail, postcards get Marketing Mail letter pricing.

In general, "letters" are any cards or envelopes that:

• are rectangular in shape; and,
• are at least 3½ inches tall but no more than 6⅛ inches tall; and,
• are at least 5 inches wide but no more than 11½ inches wide; and,
• are at least 0.007 inches thick but no more than ¼ inch thick.

Here's an exception: small cards that no more than 4¼ inches tall and 6 inches wide and are mailed at First Class retail (regular mail) are classified as "postcards" and get a lower postage rate. The same is true for cards up to 6" x 9" when using Presorted First Class.

But for Marketing Mail bulk mail and other mail classes, small cards are classified as letters and require the same postage as other pieces in the letters category. That means that for Marketing Mail, there is no postage advantage to sending the small postcards instead of larger cards (5½" x 8½" half-sheet cards, for example), and larger cards are more likely to be noticed by the recipient.

A surcharge is added to your postage rate if your letter-sized mail cannot be sorted on automated equipment. Letters that cannot be sorted on automated equipment are called "nonmachinable letters", and those that can are called "machinable letters".

Normal letter-sized mail is classified as "machinable", and there is no surcharge.

Probably the most common way that a letter becomes nonmachinable is to have an unusual shape. Anything that's not rectangular is nonmachinable. And, a rectangle that is vertical, square, or too narrow is nonmachinable.

To check the shape of your letter, divide the length of the piece by its height. If the result is less than 1.3, or more than 2.5, the piece is nonmachinable.

Exception: Mailers may use an 11-inch by 8.5-inch sheet to create four identical-size postcards, each 5 1/2" x 4 1/4", and those are considered machinable even though the length divided by height is only 1.294.

• it weighs more than 3½ ounces (not applicable to First Class, which can only be 3½ ounces or less); or,
  
  
• it is not rectangular (except that cards may have rounded corners with a radius of no more than 1/8 inch and still be machinable); or,
  
  
• its length divided by its height is less than 1.3 or more than 2.5 (except that postcards that are 5 1/2" x 4 1/4" are machinable); or,
  
  
• it is polybagged, polywrapped, or enclosed in any plastic material; or,
  
  
• it has clasps, strings, buttons, or similar closure devices; or,
  
  
• it contains items such as pens, pencils, or loose keys or coins that cause the thickness of the mailpiece to be uneven; or,
  
  
• it has staples or saddle stitching, except that these may be used on booklet-type mailpieces to join the bound edge (spine), if the inserted staples or stitching is parallel the bound edge, seat tightly and securely, and have no protrusions that might impede or damage the mail or mail processing equipment; or,
  
  
• it is too rigid to bend around an 11" diameter turn in the sorting machines; or,
  
  
• it has a thickness under 0.009 inch, (only applies for pieces at least 4¼" high or 6" long); or,
  
  
• it has the address running in the same direction as the shorter side instead of the longer side; or,
  
  
• it is a folded self-mailer and has a fold that is perpendicular to the address, unless it meets special USPS folding and sealing requirement for pieces with perpendicular folds; or,
  
  
• it is a booklet and does not meet USPS binding and sealing requirements for booklets; or,
  
  
• its outside surface is not made of paper (except for the window in a window envelope); or,
  
  
• it is something other than an envelope, folded self-mailer, booklet or card, and it is not sealed in a manner specified by USPS; or,
  
  
• it is made of paper that is too flimsy to pass through the sorting machines reliably.

There are specific details for many of these conditions. Contact us or your postmaster if you need further details.

Also, there is a separate set of restrictions for envelopes carrying a CD or DVD. We can provide those details as can your postmaster.

A flat is a mail piece that is too big in at least one direction to be a letter, but not too big.

That means it must meet AT LEAST ONE of the following:

• more than 6⅛ inches on its shorter side; or,
  
• more than 11½* inches on its longer side; or,
  
• more than ¼ inches thick.
  * 10½ inches for Every Door Direct Mail.

AND, meet ALL of the following:

• no more than 12 inches on its shorter side; and,
• no more than 15 inches on its longer side; and,
• no more than ¾** inches thick.
  ** 1¼ inches for Periodicals class nonmachinable pieces.

All flats (except Periodicals class nonmachinable flats) must be flexible enough to go through automated sorting equipment, but not be too "flimsy" (which is called "deflection"). There are complicated tests for flexibility and deflection, but in general, a normal card or envelope will be ok.

If you're mailing anything in a box, it will not be flexible enough to be a flat, and will be classified as a parcel.

A parcel is anything that is too large or rigid to be a flat.

To be sent though the mail, a parcel other than a roll or a tube must have one surface that is large enough to hold the required USPS shipping label and all other required markings (like hazardous materials markings), without any labels or markings folding over an edge. Typically, that means it must be at least 6" x 4" to hold the preferred standard USPS shipping label, though there are smaller acceptable shipping label sizes.

Parcels are divided into two categories: machinable and nonstandard. There is also a special category called Marketing Mail Marketing Parcels, which can be either machinable or nonstandard.

You may have heard of "irregular parcels" and "nonmachinable parcels". Those categories were combined into the "nonstandard" category in November, 2024.

A machinable parcel is a box that is rectangular in shape (not including tubes, rolls, etc.) so that it can be processed on automated sorting equipment, and that is too large or rigid to be a flat. Soft goods wrapped in paper or plastic bags, and enveloped printed matter, can be machinable under certain conditions. Check with your postmaster.

To be a machinable parcel, the piece must meet ALL of the following:

• its length must not be more than 22", and,
• its width must not be more than 18", and,
• its height must not be more than 15", and,
• it must weigh no more than 25 pounds.

Exception: For Marketing Mail parcels only, a parcel must weigh at least 3.5 ounces to be machinable, in addition to meeting the other size requirements.

A nonstandard parcel is any parcel that does not meet the size or weight requirements to be a machinable parcel.

A parcel can also be nonstandard because of its shape or contents. The following are nonstandard regardless of size or weight:

• Cylindrical tubes or rolls.
• A can or wood or metal box.
• A parcel containing more than 24 ounces of liquid in glass containers.
• A parcel containing one or more gallons of liquid in metal or plastic containers.

A marketing parcel is a special category of Marketing Mail parcels that contains product samples or other promotional items that are sent out in bulk.

Marketing Mail Marketing Parcels have special lower rates for commercial shippers, and even lower rates for authorized nonprofit shippers.

The purpose of the mailing must be to encourage recipients to purchase a product or service, make a contribution, support a cause, form a belief or opinion, take an action, or provide information to recipients. Marketing Mail Marketing Parcels cannot be used for fulfillment, in other words, shipping merchandise that customers have ordered.

Marketing parcels are required to be addressed using an "alternative" addressing format ("OCCUPANT", or the person's name plus "OR CURRENT OCCUPANT", etc.).

Marketing parcels have a maximum size of 12" by 9" by 2".

Note that there is also a category of Marketing Mail parcels called "Marketing Mail Nonprofit Parcels", which get slightly higher rates than the nonprofit rates for Marketing Mail Marketing Parcels, but which don't have to meet the same size and content restrictions.

The terminology is so confusing that even USPS sometimes gets confused. To be clear, here are the categories for Marketing Mail parcels:

• Marketing Mail Marketing Parcels - commercial rates.
  No fulfillment, must use "Or Current Resident" or similar, no more than 12" x 9" x 2"
  
  
• Marketing Mail Marketing Parcels - nonprofit rates.
  No fulfillment, must use "Or Current Resident" or similar, no more than 12" x 9" x 2"
  
  
• Marketing Mail Nonprofit Parcels - nonprofit mailers only.
  Can be used for fulfillment, does not need "Or Current Resident", no more than 108" length plus girth.

For more information about Marketing Mail Parcels, see our bulk parcels guide.

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