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Is it a Letter or a Flat?

Bulk Mail 101
Help Topics:


Getting Bulk Rate Discounts
Barcoding and CASS Certification
Move Update (NCOA) Requirement
Is it a Letter or a
Flat?

Bulk Mail Postage Prices
Presorted First Class Prices



If you are preparing bulk mail, the very first thing you have to know is what mail classification your mail pieces fit into. Not only do these classifications determine postage, they are actually routed differently as they travel across the country, since different mail sorting centers handle different types of mail. Software like Postage $aver automatically determines the correct routing once you specify the type of mail you are sending.

The seven classifications are:
Unfortunately, the terms used for each of these classifications 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 classifications are based on the size and shape of the mail piece. It has absolutely nothing to do with the content.



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What is a letter?

Very simply, for sorting purposes, a letter is ANY piece of mail shaped like a rectangle or square and ALL of the following are true:
  • Its height is at least 3 1/2 inches and no more than 6 1/8 inches; AND,
  • Its width is at least 5 inches and no more than 11 1/2 inches; AND,
  • Its thickness is at least 0.007 inches and no more than 1/4 inch.
So, if you draw a rectangle that is 6 1/8" by 11 1/2", and you can set your mail piece down on the rectangle and not have any of it outside of the rectangle, then it is a letter (as long as it is not too small and is not more than 1/4" thick).

While any piece of mail that fits this size description is considered to be a letter for sorting purposes, there is a special price only in First Class mail for postcards smaller than a certain size. (There is not a special postcard price in Standard Class.) See the section on postcards below for the size requirements.

Letters that can be sorted on automated equipment get better postage prices than those which can't. Such pieces are called "machinable" letters. A piece that meets the size requirements above is considered a machinable letter if it also:
  • Weighs no more than 3.3 ounces; AND,

  • Is rectangular (except that letter-sized cards may have rounded corners with a radius of no more than 1/8 inch); AND,

  • Does NOT have any of the following characteristics:

    • Its length divided by its height is less than 1.3 or more than 2.5 (a square piece would have a length divided by height of 1, meaning that square pieces are not 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 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 4-1/4 inches high or 6 inches long); OR,

    • It has the address running in the same direction as the shorter sides instead of the longer sides; 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.
A letter that is not "machinable" is called "nonmachinable".

For special barcoded (automation) prices, letters must meet additional requirements beyond those for machinable letters. If you are mailing something other than a standard envelope or card, and you want to use barcoded prices, check with your postmaster for eligibility. Barcoded pieces can weigh up to 3.5 ounces.


What is a flat?

A flat is any rectangular or square mail piece that is too big in at least one direction to be a letter.

That means that you have a flat if ANY of the following are true:
  • Its shorter side is more than 6 1/8", OR,
  • Its longer side is more than 11 1/2", OR,
  • Its thickness is more than 1/4".
Well, that's not quite true...

There are also maximum dimensions that the piece cannot exceed. To be a flat, ALL the following must also be true:
  • No side is longer than 15", AND,
  • The shorter side is 12" or less, AND,
  • The piece is no more than 3/4" thick, AND,
  • The piece weighs no more than 13 ounces for first class, or is under 16 ounces for standard class.
(Periodical class mailers see the end of this section for additional permissible dimensions.)

Under a recent change to the USPS rules, all flats (except for some periodical class flats) must now be flexible. There is a complicated test for what is flexible enough. If you aren't sure whether your piece is flexible enough to be a flat, check with your postmaster. If you're mailing a CD or DVD, or anything in a rigid box, it will not be flexible enough to be a flat, and you will pay significantly higher prices than for flexible pieces. (Actually, if you are mailing a CD or DVD, you should shop for special envelopes that hold the disc in the middle and allows it to be mailed as a letter.)

Pieces that are too rigid to be classified as flats generally must be sent as parcels.

For special barcoded (automation) prices, flats must meet additional requirements. If you are mailing something other than a standard envelope or card, and you want to use barcoded prices, check with your postmaster for eligibility.

Periodical Class mailers of flats must contend with one more classification. There are now two types of automated flats sorters in use. Periodical Class flats that can be sorted on the newer machines are considered to be "machinable" flats and get better prices than pieces that must use the older machines. Those sorted on the older machines are called "nonmachinable" flats, even though the USPS uses machines to sort them.

Periodical Class machinable flats must meet all the special requirements for barcoded flats, although they do not actually have to be barcoded. If they are barcoded, they get the best prices, but if they are not barcoded, they still get better prices than nonmachinable pieces.

Periodical Class nonmachinable flats get higher prices than machinable flats, but:
  • They are allowed to be as thick as 1 1/4" (compared with 3/4" for machinable flats),
  • They are allowed to weigh up to 4.4 pounds (compared with 20 ounces for machinable flats), AND
  • They DO NOT need to meet flexibility requirements!
Periodical nonmachinable flats MAY be barcoded for better prices.


What is a postcard?

For sorting purposes, all cards are considered to be letters or flats, depending on their size as described above.

However, cards below a certain size qualify for a special First Class postcard postage price. If you are sending barcoded postcards smaller than the size shown below, and you have at least 500 pieces in each mailing, you should use the First Class postcard price instead of the Standard Class letter price (except for nonprofit mailers, who still receive a better discount using Standard Class nonprofit prices.)

Cards are eligible for the First Class postcard price if:
  • The card is rectangular in shape; AND,
  • Its height is at least 3 1/2 inches and no more than 4 1/4 inches; AND,
  • Its width is at least 5 inches and no more than 6 inches; AND,
  • Its thickness is at least 0.007 inches and no more than 0.016 inches.
Cards that are eligible for the First Class postcard price are sorted the same way First Class letters are sorted and use tray tags that say "LTR".

If your cards are larger than the size shown above, you should use the Standard Class letter or flat price, depending the size of the card.


What is a machinable parcel?

A machinable parcel is a mail piece that is regular in shape (not including tubes, rolls, etc.) so that it could be processed on automated sorting equipment, and that is too large or rigid to be a flat.

To be a machinable parcel, the piece must meet ALL of the following:
  • Its length must be at least 6" but not more than 27", AND,
  • Its height must be at least 3" but not more than 17", AND,
  • Its thickness must be at least 1/4" but not more than 17",AND
  • It must weigh at least 6 ounces but not more than 25 pounds.
Note that if the piece is exactly 1/4" thick, it must be at least 3 1/2" high.

Some pieces that weigh less than 6 ounces can also be classified as machinable parcels if they meet certain special dimensional and packaging requirements. And, there are some irregularly shaped parcels, like those containing soft goods, that can also be classified as machinable parcels. If you have mail pieces like these, check with your postmaster for details.


What is an irregular parcel?

An irregular parcel is pretty much everything that is not a letter, flat, or machinable parcel. The classification includes rolls and tubes up to 26" long, and anything else that is too big or too irregularly shaped to be sorted on automated equipment.

If you have something with a strange shape, or that is larger or heavier than the categories above, check with your postmaster to see if it is an irregular parcel.


What is a marketing parcel?

Marketing parcels are a special category used in Standard Class mail only. A marketing parcel is either a machinable or irregular parcel, weighing less than 16 ounces, containing information and/or product samples whose purpose is 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 parcels are required to use an alternative addressing format ("OCCUPANT", or name plus "OR CURRENT OCCUPANT"). Marketing parcels have a maximum size of 12 inches by 9 inches by 2 inches thick.


What is an outside parcel?

An outside parcel is a mail piece that exceeds the size limitations for machinable parcels. The classification also includes rolls and tubes longer than 26 inches, metal-band strapped boxes, metal and wood boxes, cartons containing containing 1 gallon or more of liquid (or 24 ounces or more if it is in glass containers), and other similar pieces. Basically, these are pieces that you would think of as motor freight.

If you have something that comes close to any of these descriptions or just doesn't seem to fit any of the other classifications, check with your postal service office to see if it is an outside parcel.

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