What is the smallest size envelope that can be mailed?

Did you know mail is classified by shape and the way it is prepared?

What is the smallest size envelope that can be mailed? The classification is based on how efficiently it can be sorted through Postal Service equipment.

the smallest size envelope that can be mailed, blue 3.5 x 5 envelope diagram the largest size envelope that can be mailed, pink 6.125 x 11.5 envelope diagram

To be eligible for mailing at the price for letters:

So...what is the smallest size envelope that can be mailed?

It must be rectangular.

A minimum of 3.5 inches in height by 5 inches in length and at least .007 inches thick.

A maximum of 6.125 inches in height by 11.5 inches in length and .25 inches thick

Not more than 3.5 ounces (First-Class Mail letter-size pieces over 3.5 ounces pay flat-size prices

Rectangular, with four square corners and parallel opposite sides


Length is the side parallel to the address.

Height is the side that is perpendicular to the length.

How can I measure 0.007 inches? An index card is a great comparison 

The Postal Service classifies letters and cards into one of three categories:

Machinable, Non machinable, and Automation.

Machinable Letters and Cards: If your letters and cards have an address and can be processed on Postal Service equipment, your mail is “machinable” and eligible for “presort” prices [more info found here]. Machinable mailpieces must meet standards, including size, shape, and weight. (shown above)
Non machinable Letters: 
First-Class Mail letters that have a non machinable characteristic will have to pay a surcharge. The Postal Service charges extra postage because mailpieces that are rigid, square, or unusually shaped, often jam postal equipment and are difficult to process. Examples of a non machinable letter:

An aspect ratio (length divided by height) of less than 1.3 or more than 2.5 (a square envelope has an aspect ratio of 1 and is non machinable).

It is more than 4.25 inches high / more than 6 inches long / is less than 0.009 inch thick.

Has clasps, strings, buttons, or similar closures.

It is too rigid.

It has a delivery address parallel to the shorter side

Contains items such as pens, pencils, or keys that create an uneven thickness.

Letters with nonpaper surfaces, other than envelope windows or attachments that are allowed

Maximum weight: First-Class Mail and Periodicals non machinable letters: 3.5 ounces  |  Standard Mail non machinable letter: less than 16 ounces.

Folded self-mailers must be sealed or they will be subject to the nonmachinable surcharge. Use wafer seals or tabs to close the open sides

Some examples of a non machinable letter:

non machinable letters: colorful square envelopes and policy envelopes

Automation Letters and Cards: Lower prices are available for letters and cards that are compatible with the Postal Service’s automated processing equipment. To qualify for automation prices, your letters and cards must conform to specific criteria, including:

A minimum number of pieces (200 pieces for Standard Mail, 500 pieces for First-Class Mail).

An Intelligent Mail barcode encoded with the correct delivery point routing code on each piece in the address block or in the barcode clear zone

An address list that has been checked and CASS-certified within the last 180 days.

A complete address with ZIP Code or ZIP+4 code.

A specific shape and size.

No more than 3.5 ounces for automation Standard Mail and Standard Mail Carrier Route.

No polywrap, polybag, shrinkwrap, clasps, strings, buttons, or similar closure devices

We hope you've enjoyed learning about the smallest size envelope that can be mailed!

[ Shop for envelopes here! ]

all information directly from USPS. For more information on non letter pricing click here.

Understanding the Smallest Size Envelope That Can Be Mailed

When it comes to mailing envelopes, size matters. The smallest size envelope that can be mailed is known as a "minimum size envelope." These envelopes typically measure 3.5 inches by 5 inches, making them perfect for small notes, RSVP cards, and other compact items. Despite their small size, these envelopes are fully mailable and can be sent through the postal system without any issues. They are a popular choice for sending out small, personalized messages or invitations.

The Benefits of Using Small Envelopes

Small envelopes offer several benefits, including cost-effectiveness and versatility. They are often more affordable to purchase and mail than larger envelopes, making them a budget-friendly option for sending out small items. Additionally, their compact size makes them ideal for creating a personalized and intimate experience for the recipient. Whether used for special occasions or everyday correspondence, small envelopes can make a big impact.

Use Cases for Small Envelopes

Small envelopes have a wide range of use cases, from wedding RSVPs and thank you notes to business cards and gift cards. Their diminutive size makes them perfect for enclosing small, delicate items and adding a touch of elegance to any correspondence. They are also commonly used for marketing materials, such as promotional offers and event invitations, due to their eye-catching and unique appearance.

Alternatives to Small Envelopes

While small envelopes are a popular choice for many mailing needs, there are alternatives to consider. For slightly larger items, such as photos or standard-sized letters, a larger envelope may be more suitable. Additionally, for items that require extra protection, padded envelopes or bubble mailers can provide added security during transit. It's important to consider the specific needs of the contents being mailed when choosing the right type of envelope.

Tips for Using Small Envelopes

When using small envelopes, it's important to consider the size and weight restrictions imposed by postal services. Ensuring that the contents fit comfortably within the envelope and do not exceed the weight limit will help prevent any issues during mailing. Additionally, using clear, legible addressing and proper postage will help ensure that the envelope reaches its destination without delay. Small envelopes can be a charming and practical choice for a variety of mailing needs when used thoughtfully.

Additional Considerations for Small Envelopes

When selecting small envelopes, it's essential to choose a high-quality option that offers durability and a professional appearance. Additionally, considering the color, texture, and closure style of the envelope can add a personalized touch to the mailing experience. Whether used for personal or business purposes, small envelopes can make a memorable impression when chosen and utilized thoughtfully.


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