Flats USPS


March 9, 2024

The post office is the typical place to send letters or postcards. However, sometimes, you must send documents that won’t fit in a standard letter envelope. 

For this situation, USPS (United States Postal Service) provides large envelopes that can contain larger-sized documents. The Postal Service calls these large envelopes “flats.” 

What are large envelopes or flats? What can you mail using these mail containers, and how much do they cost

This article explains what flats are and how people use these large envelopes to mail documents. It also explains the different mailing options used to ship flats. 

Furthermore, this write-up discusses other mail options for mailing flats, including Priority Mail, Direct Mail, and First-Class Mail International. 

If you need access to contact information for over 31,330 post offices in the country, visit FindPostOffice.org. Our website can help you get the address of your local post office and assist you in managing your First-Class Mail Flat Rate deliveries. 

What Is a Large Envelope (Flat)?

It may sound obvious, but the U.S. Postal Service calls a large envelope a flat. You may think, are there any envelopes that aren’t flat? However, USPS has specific criteria for placing an envelope size under the “flat” category. 

Surprisingly, USPS doesn’t consider flats because these document containers are literally flat but because of the following reasons.

A Flat As Defined by USPS

USPS has its definition of the word “Flat.” This official definition determines whether the postage needed for a parcel is for letters or large envelopes. Here are the criteria for USPS to consider a parcel as a “flat.” 

  • The envelope shouldn’t exceed the maximum size of 15in long x 12in high x 3/4in thick.
  • The weight shouldn’t exceed the maximum 13oz.
  • The envelope’s shape must be rectangular, which includes square-shaped parcels. 

Say the mailpiece exceeds a large envelope’s maximum weight and dimensions. In that case, USPS will consider them packages and may be shipped as a USPS Priority Mail item. 

Large Envelopes Technically Need to Be Mailed, Not Shipped

Mailing and shipping both mean sending something via the Postal Service. However, the two have slight differences, especially on the type of item shipped via USPS. 

Mailing is when you’re sending small parcels like letters and large envelopes. On the other hand, shipping is when you send packages or mail items not considered letters or envelopes. 

So, if you’re going to use exact technical terms, you mail large envelopes, and you ship packages. 

Sizes for Large Envelopes and Flats

Here is a table showing the parcel dimensions under the USPS flat category. 

MeasurementMore thanMaximum
Height6 1/8 inches12 inches
Length11 1/2 inches15 inches
Thickness1/4 inch3/4 inch


The table above shows the minimum and maximum sizes for large envelopes or “flats.” If your parcel falls under the minimum size, the item is considered a letter and will have letter-size rates. 

However, if the parcel exceeds the maximum limit, it’s considered a package. USPS provides specific shipping services for parcels that are considered packages. 

What Are First-Class Mail Flats Retail Rates?

The following shows the First-Class Mail rates for flat-rate envelopes. Note that prices are as of 2023. Rates may increase at a future date.

USPS large envelope flat servicePrice rates as of July 2023
Priority Mail Express Flat Rate envelope$28.75
Priority Mail Express Legal Flat Rate envelope$28.95
Priority Mail Express Padded Flat Rate envelope$29.45
Priority Mail Flat Rate envelope$9.65
Priority Mail Legal Flat Rate envelope$9.95
Priority Mail Padded Flat Rate envelope$10.40

First-Class Mail is the cheapest mailing option in USPS’ list of services. The affordability of First-Class Mail is suited for the type of mail items that can be delivered via this service. 

You can send letters, postcards, large envelopes, and lightweight packages via First-Class Mail. You can take advantage of the low rates if your letters weigh less than 3.5oz and your packages weigh less than 13oz. 

Many small businesses and start-up eCommerce sites can benefit from USPS’ First-Class Mail. Additionally, there are items that you must mail via First-Class Mail or Priority Mail. 

  • Any handwritten or typewritten letter
  • Bills, credit cards, or invoices
  • Personal letters or correspondence
  • Business correspondence
  • Any mailable item sealed in a First-Class Mail envelope

First-Class Mail International (FCMI)

When you need to mail letters or flats to addresses outside the country, FCMI is one economical way to send your mail to over 180 countries worldwide, including Canada, Great Britain, and Australia. 

You can send flats internationally using this service. However, you may need to pay the corresponding rates for flat-size mailpieces. The cost to mail a flat via First-Class Mail International is $1.50 or one Global Forever stamp as postage

Direct Mail Flat – Flats From the Perspective of a Direct Mail Professional

Direct Mail is an advertising strategy where you send an advertisement, promotion, or announcement directly to a person’s address. This strategy depends on the population demographics and the data you get on people’s consumer habits in a specific area. 

The aim of sending direct mail pieces is to hook potential customers into your service and convert them into paying customers.

From the direct mail industry perspective, “flats” can include self-mailers, mail that doesn’t require an envelope, like a postcard. 

A direct mail professional sees self-mailers as flats. They cost less than traditional direct mail items. However, not all people will appreciate this kind of mail. 

Open-Side and Open-End Envelope Flats

Many in the direct mail industry classify envelopes into open-end and open-side envelopes. 

The open-end envelope has an opening flap at the short end of the envelope. Meanwhile, open-side envelopes have an opening flap at the long side of the envelope. 

Choosing the type of envelope based on where the opening is might be trivial. However, a direct mail professional sees a parcel container and determines how it complements the advertising material, adds aesthetics, and ensures machinability. 

Furthermore, the direct mail professional considers a particular envelope type’s postage rate or shipping cost. 

Are All Flats Created Equal?

Not all large envelopes of flats are created equal. USPS has specific requirements for parcels accepted as flats, which may lead to confusion if not explained adequately. 

Many kinds of envelopes are in the market and could be deemed as flats by people. However, a significant determiner is the machinability of the item. So, what makes a letter machinable or nonmachinable?

Machinability is an item’s eligibility to be processed or presorted by a USPS automated machine. Suppose an envelope is nonmachinable despite being in the right weight or dimension. In that case, it is not similar to flats that USPS can process. This simple example shows that not all flats or large envelopes are created equal. 

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

Your local post office categorizes items that can be sent via First-Class Mail service. You can mail four items through this service: postcards, letters, flats, or parcels. 

What Does USPS Consider to Be a Postcard?

The definition of a postcard is mail that you send without using an envelope. The first postcard issued by a government was the Austrian Korrespondenz Karte in 1869. 

USPS also defines postcards as correspondence sent without an envelope, but it doesn’t end there. The Postal Service has other requirements before a mailpiece is considered a First-Class Mail postcard. 

  • It is rectangular
  • It is at least 3.5in high x 5in long x 0.007in thick
  • It doesn’t exceed 4.25in high x 6in long x 0.016in thick

If any of these three don’t match the mail item, it won’t be considered a postcard. 

What Does USPS Consider to Be a Letter?

A letter is a message sealed in an envelope and mailed to a recipient. However, USPS adds to the definition of letters as messages recorded in or on a “tangible” medium. 

This definition implies that paper is not the only medium for letters. USPS considers Items like different data storing devices like recording disks or magnetic tape storing personal correspondence as letters.

USPS states the following are item requirements to be considered a letter:

  • The mailpiece must not exceed 11 1/2in long x 6 1/8in high x 1/4in thick
  • The maximum weight must not exceed 3.5oz
  • Must be machinable

If the mailpiece doesn’t meet the requirements, it will not be considered a letter. 

If you still want to mail items that don’t fit this criteria, you may pay extra for postage letter price or surcharges.

What Is the Difference Between a Flat and a Parcel?

As previously mentioned, a flat is another term for a large envelope. USPS considers mailpieces as flats if they exceed the weight and dimensions of a letter. 

A flat is a large envelope. Anything larger than a flat will be considered a parcel. USPS also considers items larger than a flat as a parcel. However, there are additional requirements. 

  • The mailpiece must be at least 6in long x 3in high and 0.25in thick.
  • No mailpieces can be bigger than 108in of combined length and girth.
  • The mailpiece must not exceed the maximum weight of 70lbs.

What Is a Marketing Parcel?

Businesses and other stores can purchase the Marketing Parcel service to ship flyers, catalogs, product samples, and advertisements. The marketing package can contain these aforementioned mailable items but with restrictions imposed by the U.S. government. 

Furthermore, the dimensions of a marketing parcel are 12in in length x 9in in height x 2in in thickness. It also has a maximum of 16oz in weight. 

What Are USPS Size Categories for Bulk Mail?

USPS defines bulk mail as a quantity of mail that is enough to receive mail discounts. Bulk mail is presorted by ZIP (Zone Improvement Plan) code. USPS considers a collection of mail as bulk mail if it meets the following numbers:

  • 50 mailpieces for Parcel Posts
  • 200 mailpieces for Marketing Mail
  • 300 mailpieces for Presorted Bound Printed Matter
  • 500 mailpieces for Presorted First-Class Mail
  • 500 mailpieces for Presorted Library Mail
  • 500 mailpieces for Presorted Media Mail

USPS Flats Sequencing System Comes Under Scrutiny

USPS provides a flats sequencing system (FSS), a mail sorting system for flat-sized mail, including catalogs and magazines, into delivery point sequences. 

The FSS is a huge system with massive equipment half the width of a football field. It has several crucial subsystems to prepare a USPS mail processing plant before delivery. 

The price tag for an FSS pre-production is $1.3 billion. USPS worked to reduce this price, but it still landed on an $874 million bill. This amount is to produce and deploy 100 FSS units at 32 locations in the United States. 

However, the declining mail volume was so significant that the billion-dollar investment attracted questions regarding its appropriateness. 

The Standard Mail Flats volume between 2008 and 2013 declined from around 10 billion to 5.6 billion pieces. 

Can You Use Package Services to Send Flats With USPS?

Package Service was previously called Fourth-Class Mail. This service provides a cheap ground shipping option for merchandise, catalogs, and different media materials. 

However, you can’t mail personal correspondence via Package Services. Thus, you can’t send flats with personal letters via this USPS mailing option. 

Do Flats Have USPS Tracking?

First-Class Mail letters, postcards, and flats don’t have USPS Tracking services. However, these items can use additional services, like a barcode for scanning. These additional services have some tracking information you can use to monitor your mail’s progress. 

Are USPS Flats Machinable?

This article previously mentioned that a mailpiece is machinable if USPS’ automated equipment can process it. 

You can determine if your flat is considered machinable. Note that if your mailpiece is nonmachinable, you may need to pay a surcharge for the additional processing load for the postal worker. 

The following attributes make flats machinable:

  • Length is more than 11.5in but doesn’t exceed 15in. 
  • Height is more than 6.125in but doesn’t exceed 12in.
  • Thickness is more than 0.25in but doesn’t exceed 0.75in.
  • Weight is more than 3.5oz but doesn’t exceed 16oz. 
  • The flat must be rectangular or have four right-angled corners. A square-sized flat is considered to be machinable. 

If you need a helpful website with a vast database of post offices in the country, visit FindPostOffice.org. Our website provides access to updated contact information of over 31,330 post offices and USPS retail facilities in the United States. 


  1. First-Class Mail Fact Sheet
  2. What is a Large Envelope (Flat)?
  3. Pricelist
  4. What are Package Services?
  5. Direct Mail: What it is, How it Works, In Practice
  6. 201 Quick Service Guide
  7. Sizes for Postcards
  8. 240k Quick Service Guide
  9. What are Package Services?