Is a 6×9 envelope considered a flat?

A 6×9 envelope is a common envelope size used for mailing letters, documents, and other flat items. But is it technically considered a “flat” by the United States Postal Service (USPS)? This is an important question for understanding shipping requirements, postage rates, and mailing regulations.

What is a 6×9 envelope?

A 6×9 envelope has exterior dimensions of 6 inches by 9 inches. This is a standard envelope size that is widely used for both personal and business mailings.

Some key features of 6×9 envelopes:

  • Height: 6 inches
  • Width: 9 inches
  • Fits standard 8.5 x 11 inch papers when folded
  • Used for letters, invitations, bills, documents, etc.
  • Available in white wove, kraft, colored, and window styles
  • Can have various flap designs like side seam, diagonal seam, wallet, etc.

The 6×9 size is convenient because it is large enough to enclose papers folded in half while still being easy to handle and mail. The postal service and envelope manufacturers produce many products catered to this popular size.

USPS shape-based mail classifications

The USPS categorizes mail based on shape for purposes of mail processing, sorting, and postage pricing. The main shape categories are:

  • Letters – Rectangular with certain dimension ratios
  • Flats – Flexible envelopes larger than letter size
  • Parcels – Rigid boxes and packages

These broad categories are further divided into more specific sub-groups. The shape determines factors like machine sortability, if minimum dimensions apply, and pricing thresholds.


For First-Class Mail, letters must have dimensions of:

  • Minimum: 5 inches long, 3.5 inches high, 0.007 inch thick
  • Maximum: 11.5 inches long, 6.125 inches high, 0.25 inch thick
  • Length plus height must be 6 inches up to 12 inches
  • Aspect ratio between 1:1.3 and 1:2.5

Postcards are also classified as letters if they meet the minimum and maximum sizes. Square envelopes with equal length and height can qualify as letters too.

Large envelopes (flats)

A flat is defined by USPS as:

  • More than 11.5 inches long OR more than 6.125 inches high
  • No more than 12 inches high OR 15 inches long
  • 0.25 inch thick maximum

Common examples are large envelopes, catalogs, padded envelopes, manila folders, newspapers, and magazines. Length and height cannot exceed the maximums for flats even if other dimensions meet the rules. Flats exceed at least one of the letter dimensions but must not exceed the flat maximums.


Parcels are packages like boxes that are rigid, uniformly thick, and do not meet letter or flat dimensions. Minimums for parcels are:

  • 3.5 inches high
  • 5 inches long
  • 0.25 inch thick

There is no maximum size for parcels. Common parcel shapes are cubes, rolls, cylinders, polybags, and triangles.

Does a 6×9 envelope qualify as a flat?

A standard 6×9 envelope with a maximum thickness of 0.25 inches falls within the dimensional guidelines for a flat according to USPS regulations. The length and height are both under the 12 inch and 15 inch maximums respectively. It exceeds the 6.125 inch height limit for letters.

The Postal Service explicitly uses 6×9 envelopes as an example of a flat when describing the characteristics of flats. Based on the USPS shapes criteria, a typical 6×9 envelope is classified and processed as a flat.

USPS pricing for 6×9 envelopes

Mail shape also determines the postage price. As a flat, a 6×9 envelope will be priced according to the USPS commercial flats pricing program. The exact rate depends on factors like:

  • Mail class – First-Class vs. Standard
  • Weight – Ounces
  • Automation compatibility – Ability to be machine sorted
  • Destination – Domestic or international
  • Services – Certified mail, return receipt, etc.

Some example prices for 6×9 envelopes at the time of writing:

Mail Type Weight Price
First-Class Mail Letter 1 oz. $0.58
First-Class Mail Flat 1 oz. $1.00
First-Class Package Service 2-3 oz. $2.95
Standard Mail Flat 3.3 oz. $0.44

These prices demonstrate the premium charged for flats versus letters. Even a 1 ounce flat costs considerably more to mail than a 1 ounce letter. Heavier 6×9 envelopes may be better priced under packages rather than flats.

Dimensional weight pricing

If the cube (volume) of a 6×9 envelope is greater than 1 cubic foot, the USPS will use dimensional weight pricing for postage instead. This charges for cubic space taken up in transport and processing rather than just weight.

Dimensional weight is calculated as:

  • Length x Width x Height (in inches) / 194 = Dimensional Weight (in pounds)

A typical 6×9 envelope has a dimensional weight of:

  • 6 x 9 x 0.5 (inches) / 194 = 0.14 pounds

This comes out lower than the actual weight, so dimensional pricing would not apply. But for a thicker, heavier 6×9 envelope, dimensional weight could kick in resulting in a higher postage price.

Qualifying for flat/large envelope prices

To qualify for the flats/large envelope pricing, 6×9 envelopes must meet USPS automation compatibility standards which include:

  • Minimum size of 6 inches high x 9 inches long x 0.009 inch thick
  • Maximum size of 12 inches high x 15 inches long x 0.75 inch thick
  • Weight maximum of 16 ounces
  • Aspect ratio between 1:1.3 and 1:2.5
  • Must be flexible and uniformly thick
  • Addressed with OCR readable font and layout

Meeting automation standards gets the lowest flat rates. Non-automation flats may incur additional charges or require special handling.

Special case mailers

Sometimes 6×9 envelopes are used as special case mailers that do not seal on all sides like standard envelopes. Examples are:

  • Catalog envelopes with openings for content visibility
  • Bangtail envelopes with extended flap lengths
  • Booklet envelopes with one short open side

These types of 6×9 mailers may still qualify as flats but could have surcharges applied in some cases. The open sides can limit their automation compatibility.

When a 6×9 envelope is not a flat

There are some cases where a 6×9 envelope may not be categorized or priced as a flat:

  • Square flap envelopes – These are treated as letters if within size limits
  • Rigid or irregularly shaped – Would not meet flexibility standards
  • Overstuffed – Thicker than 3/4 inch maximum
  • Nonmachinable criteria – Clasps, string closure, rigid contents
  • Hazardous materials – Some chemicals, batteries, etc.

If the envelope and contents do not allow for automated processing on USPS equipment, letter prices or parcel prices would apply instead.

Key takeaways

  • A standard 6×9 inch envelope meets USPS definitions for a flat.
  • Flats receive different pricing and handling vs. letters or parcels.
  • Automation compatibility is needed for the lowest flat rates.
  • If a 6×9 envelope does not meet flat requirements, letter or parcel pricing applies.
  • Dimensional weight may increase postage for large, heavy 6×9 envelopes.


In most cases, a typical 6×9 envelope used for mailing letters or documents qualifies as a flat by USPS standards. This means it will be categorized, processed, and priced as a flat versus a letter or parcel. Exceptions occur when the envelope is too rigid, thick, or irregularly shaped for automated flat sorting. To receive the large envelope/flat rates, 6×9 envelopes must be automation-compatible based on size, flexibility, thickness and readability. When in doubt, checking with the US Postal Service on an envelope’s classification is advised to avoid potential costs from incorrect postage or unmachinable surcharges.

Leave a Comment