Business Envelope Size Guide: Types of Commercial Envelopes
How This Corporate Envelope Guide Is Structured
Business envelope size directly influences postage costs and customer perception, which is why selecting the right envelope type matters for organizations.
This guide can help.
In this article, we’ll discuss the most common business envelopes used in commercial mail, including #9 and #10 envelopes, 6″ × 9″ formats, and flat envelopes.
We’ve provided a chart organizing these categories by their dimensions, discussing their use cases and any special considerations associated with them.
We then dive deep into the commercial envelope sizes, telling you exactly when to use each.
Business Envelope Size Chart:
| Business Envelope Type | Business Envelope Dimensions (inches) | USPS Mailpiece Category | Typical Use | Special Considerations |
|---|---|---|---|---|
| #9 Envelope | 3.875 x 8.875 | Letter | Reply envelopes (BREs and CREs) | Often nested inside #10 envelope |
| #10 Envelope | 4.125 x 9.5 | Letter | Statements, invoices, notices, most documents | Most common business envelope size |
| 6″ x 9″ Envelope | 6 x 9 | Letter | Brochures, marketing materials, letter-size paper folded in half | Larger than a #10, smaller than a flat |
| 9″ x 12″ Envelope | 9 x 12 | Flat | Reports, booklets, non-folded documents | Higher postage than letters |
| 10″ x 13″ Envelope | 10 x 13 | Flat | Reports, booklets, non-folded documents | Higher postage than letters |
#10 Envelopes
What Is the Size of a #10 Envelope?
The #10 envelope has a size of 4.125 × 9.5 inches.
Its dimensions are optimized to fit standard 8.5 × 11-inch sheets folded into thirds via a C-fold or Z-fold, without excess space or bending.
This makes insertion and processing highly efficient, explaining why it is the most commonly used business envelope size in the United States.
How are #10 Envelopes Folded?
These are two common ways to fold paper that will be inserted into #10 envelopes:
- C-Fold: The bottom of the sheet is folded up first, then the top folded down, creating three stacked sections resembling the letter “C” when viewed from the side. This is the most common fold for standard statements and letters.
- Z-Fold: The sheet is folded in a zigzag pattern, top to bottom to top, creating a “Z” shape.
Both fold styles are fully compatible with high-volume inserters and help maintain consistent document presentation in business mailings.
Where is the #10 Business Envelope Size Used?
The #10 envelope is widely used for:
- Invoices, statements, and transactional notices
- General correspondence and corporate communications
- Documents requiring window alignment for addresses or account information
The #10 envelope is the most common default business envelope size for commercial mail operations.
USPS Guidelines for #10 Envelopes
The USPS classifies the #10 envelope as a letter-size mailpiece when it meets specific criteria:
- Minimum dimensions: 3.5 × 5 inches
- Maximum dimensions: 6.125 × 11.5 inches
- Aspect ratio: 1.3 to 2.5, to be eligible for automation pricing
- Maximum weight: 3.5 ounces
- Thickness: 0.007–0.25 inches
Compliance with these requirements ensures that the envelope is eligible for letter rates, and is a necessary prerequisite to obtain USPS automation letter rates.
Automation refers to high-speed sorting and processing systems used by the USPS to handle large volumes of letters. Envelopes that meet automation specifications are generally processed more efficiently and often at lower postage costs than non-automation or manual mail.
#9 Envelopes
What Is the #9 Envelope’s Size?
The size of a #9 envelope is 3.875 × 8.875 inches, making it slightly smaller than the #10.
This is the reason the #9 is an effective business envelope type.
Since it fits neatly inside a #10, the #9 commercial envelope size is often sent with the #10 as a return envelope.
It can also be used as a standalone envelope for business communications, though this is less common.
Where Is the #9 Business Envelope Size Used?
Two of the major types of #9 return envelopes are:
- Business Reply Envelopes (BREs): These are used by customers to send invoices, payment slips, or forms without paying for postage themselves. Businesses are charged the cost of the envelope only when the customer sends it back to them. They are often used when obtaining a customer’s response is more important than saving money on postage.
- Courtesy Reply Envelopes (CREs): A CRE is a reply envelope that the consumer must pay postage on. CREs are frequently used when customer response is already likely.
The #9 business envelope size is also occasionally used for small notices, single-sheet forms, or invitations that need to be mailed independently or nested inside larger envelopes.
6″ x 9″ Envelopes
What Is the Size of a 6″ x 9″ Envelope?
This envelope is, as the name implies, 6″ by 9″ in size. This makes it larger than #9s and #10s, but smaller than standard flats such as 9″ × 12″s or 10″ × 13″s.
Where Is the 6″ x 9″ Envelope Used?
The 6″ × 9″ envelope type is frequently used for:
- Manuals, small booklets, or catalogs
- Brochures, flyers, or other marketing materials
- Letters that are folded in half
This business envelope size is a good choice for commercial mailing operations when content cannot fit into a #10 envelope, but the content is not large enough to necessitate the use of a flat and the incurring of higher flat postage rates.
Flats
What Are Flats?
Flat envelopes, often referred to as “flats,” are larger than standard letter-size envelopes. This business envelope size is used when documents cannot be folded to fit smaller formats.
Flats are subject to higher USPS postage costs than letters, which must be considered when designing which business envelope type to use.
What Determines if a Corporate Envelope is a Letter or a Flat?
We’ve provided a chart, helpful when evaluating commercial envelope size, to determine whether the envelopes you’re considering will be classified as letters or flats:
| Criteria | Letter | Flat |
|---|---|---|
| Minimum business envelope size | 3.5 x 5 inches | Height over 6.125 inches OR length over 11.5 inches OR thickness over 0.25 inches |
| Maximum business envelope size | 6.125 x 11.5 inches | 12 x 15 inches |
| Maximum business envelope thickness | 0.25 inches | 0.75 inches |
| Relative postage costs | Lowest cost | Higher than letter |
What Is the Envelope Size of a Flat?
Flats come in many dimensions, but two common business envelope sizes for flats are 9″ × 12″ and 10″ x 13″, offering ample space for letter-size pieces of paper, without the need to fold.
Where are Flats Used?
This commercial envelope type is often used for documents that organizations do not wish to fold. These may include:
- Business reports, multi-page statements, or training manuals
- Legal documents, contracts, or other documents that need to remain flat for presentation or scanning purposes
- Marketing materials or brochures that cannot be folded neatly into a #10 or 6″ × 9″ envelope
Flats provide flexibility for sending larger, multi-page documents without folding, but they come with trade-offs. Namely, the increase in commercial envelope size often leads to significantly higher postage costs.
Which Business Envelope Size Should I Choose?
The business envelope types that your organization uses may vary by mailing and should always be carefully determined. Here’s our input on the matter:
If you’re sending documents that can’t fit into a #10 or a 6″ x 9″ envelope, or the materials you’re sending shouldn’t be folded for reasons of presentation, then you should use a flat, either a 9″ x 12″ or a 10″ x 13″.
If you’re sending materials that are too big for a #10, but you don’t want to pay the increased postage costs of a flat, you should use a 6″ x 9″.
Finally, if you’re sending standard business mailings, most of the time, you should use a #10 envelope. It’s the standard business envelope size for a reason, as it perfectly fits a letter-size sheet of paper, folded neatly. If you need your customers to send you return mailings back, send the #10 with a #9 to be used as a return mailer.
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Frequently Asked Questions: Business Envelope Size
What is considered a business envelope?
A business envelope is any envelope commonly used for professional, commercial, or corporate mailings. This includes business envelope sizes like #10s, #9s, 6″ x 9″s, and flat envelopes like 9″ × 12″ and 10″ × 13″.
What is the most common commercial envelope size?
The most common commercial envelope size is the #10 envelope (4.125″ × 9.5″). It is widely used for invoices, statements, notices, and general correspondence and is optimized for USPS letter automation, making it cost-effective for high-volume mailings.
What business envelope type should I use for standard documents?
For most standard documents printed on 8.5″ × 11″ paper, a #10 envelope is the preferred business envelope type. It accommodates common folds, qualifies for USPS letter automation when designed correctly, and balances cost, efficiency, and professional appearance.
When should I use a flat envelope instead of a letter envelope for commercial mailings?
Flat business envelope sizes, such as 9″ × 12″ or 10″ × 13″, are typically used when documents should not be folded, when page counts are higher, or when paper stock is heavier.
What are #9 envelopes used for?
#9 envelopes are commonly used as reply envelopes, including Business Reply Envelopes (BREs) and Courtesy Reply Envelopes (CREs). They are often included inside #10 envelopes to support remittance, response, or return correspondence.
How does business envelope size affect USPS postage?
Envelope size plays a major role in USPS classification. Letter-size envelopes can qualify for lower automation postage rates, while larger or thicker mailpieces may be classified as flats and incur higher costs. Choosing the right corporate envelope size helps control postage and ensures smoother processing through USPS equipment.
Who should I partner with for commercial mailings?
If you’re looking for a knowledgeable print and mail partner with secure business mail processes, D4 Solutions is the perfect fit for you. Our workflows have a 99.99% operational accuracy rate, and are SOC 2 Type II and HIPAA audited. We accurately print and deliver your mailings to their intended destinations, on time. Reach out now to learn more about how we can help your organization.
