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#9 Envelope Size: Dimensions, Uses, and Types Explained
Last updated: October 21, 2026
The #9 envelope size is one of the most practical in business mailing. It is the standard reply envelope, designed to slip inside a #10 and land in the hands of your customer pre-addressed and ready to send back. If you have ever received a utility bill, a nonprofit appeal letter, or a subscription renewal notice with a return envelope tucked inside, that inner envelope was almost certainly a #9.
This guide covers everything you need to know: exact dimensions, how it compares to the #10, which types are available, when to use one, and how postage works.
Quick Answer
A #9 envelope measures 3 7/8 x 8 7/8 inches (3.875 x 8.875 inches). It is designed to fit inside a standard #10 business envelope (4 1/8 x 9 1/2 inches) as a reply or return piece. No extra postage is needed on the #9 when it is enclosed in a #10.
TL;DR
- Size: 3 7/8 x 8 7/8 inches
- Designed to fit inside a #10 envelope
- Primary use: reply, return, and business reply mail
- Available in white, colored, security tinted, and window styles
- No extra postage when enclosed inside a #10 mailing
- Custom printing available with PDF proof before production
| Feature | #9 Envelope | #10 Envelope |
|---|---|---|
| Width | 3 7/8 in (3.875 in) | 4 1/8 in (4.125 in) |
| Height | 8 7/8 in (8.875 in) | 9 1/2 in (9.5 in) |
| Primary role | Reply / return envelope | Outgoing business envelope |
| Fits inside | #10 envelope | Catalog or 9x12 envelope |
| Paper weight | 24 lb wove | 24 lb wove |
| Postage when mailed alone | First-Class letter rate | First-Class letter rate |
What Are the Exact Dimensions of a #9 Envelope?
A #9 envelope measures 3 7/8 x 8 7/8 inches, or 3.875 x 8.875 inches. In metric, that is approximately 98 x 225 millimeters. It qualifies as a standard USPS letter when mailed on its own, which means it moves through postal sorting equipment without any non-machinable surcharge, as long as the aspect ratio and rigidity requirements are met.
The envelope number system used in the US is not based on inches. It is a legacy naming convention where higher numbers indicate larger sizes in the correspondence range. The #9 sits just below the #10 in that sequence, which is exactly why the two work together as a pair.
| Measurement | Value |
|---|---|
| Width | 3 7/8 inches (3.875 in) |
| Height | 8 7/8 inches (8.875 in) |
| Metric (approx.) | 98 x 225 mm |
| Paper weight | 24 lb wove |
| USPS mail class | First-Class letter |
| Designed to fit inside | #10 envelope (4 1/8 x 9 1/2 in) |
How Does a #9 Envelope Compare to a #10?
The #9 and #10 envelopes work as a matched pair in business mailing. The #10 is the outgoing envelope you mail to your customer, and the #9 is the reply envelope tucked inside it. The #9 is about 1/4 inch narrower and slightly shorter than the #10, giving it enough clearance to slide in neatly without jamming.
The critical difference is in their roles, not just their dimensions. When a #9 is enclosed in a #10, no additional postage is required on the #9. The outer envelope's stamp or meter mark covers the full mailing. The recipient then uses the pre-addressed #9 to send something back, at which point they apply their own postage, or you set it up as Business Reply Mail so they send it back postage-free and you pay only for returned pieces.
The #10 is the most widely used business envelope in the US. It holds a standard 8.5 x 11 sheet folded in thirds. The #9 holds a narrower insert, typically a folded payment stub, a reply card, or a single folded sheet, and is not intended as the primary outgoing mailer for a full letter.
If you are working on larger outgoing mail pieces, you may also want to look at #10 envelopes for your outer mailing and #9 envelopes for the reply piece inside.
What Is a #9 Envelope Used For?
The #9 envelope has a clear job: it is the reply piece. Businesses and nonprofits use it whenever they want the recipient to send something back, whether that is a payment, a donation, a signed form, or a renewal slip. Because it fits cleanly inside a #10, it adds almost no extra weight to the outer mailing and does not require separate postage until the recipient sends it back.
Common uses for a #9 envelope include:
- Invoice reply envelopes enclosed with billing statements so customers can return a payment stub
- Nonprofit donation appeals where donors mail back a gift along with a pledge card
- Subscription and membership renewals that include a tear-off reply form
- Business Reply Mail (BRM) programs where the recipient returns the envelope postage-free
- Insurance and financial correspondence where signed forms need to be returned
- Government and compliance mailings requiring a signed reply or acknowledgment
- Direct mail campaigns with a response mechanism included in the package
The #9 is less commonly used as a standalone outgoing envelope, but it can be mailed on its own for lighter correspondence when a smaller footprint is preferred.
What Types of #9 Envelopes Are Available?
Not all #9 envelopes are the same. The differences in sealing method, color, window placement, and interior tinting affect how the envelope performs in each use case. Here is a breakdown of the main types available at Business Envelopes.
| Type | Sealing | Best For | Key Feature |
|---|---|---|---|
| White Wove | Gummed flap | General business reply | Clean, professional look |
| White Window | Gummed flap | Remittance, billing reply | Address shows through window |
| Security Tinted | Gummed flap | Checks, financial docs | Interior pattern blocks contents |
| Security Tinted Peel and Seal | Peel and seal | High-volume secure mailings | Easier sealing, security interior |
| Security Tinted Window | Gummed flap | Secure reply with address window | Window plus tinted interior |
| Colored / Starburst | Peel and seal | Direct mail, seasonal campaigns | Eye-catching color options |
| Colored Wove | Gummed flap | Color-coded reply mailings | Assorted wove colors |
White #9 Envelopes
The standard white #9 is the most common choice for business reply mail. Made from 24 lb wove paper, it has a clean, professional appearance that works well for invoices, statements, and formal correspondence. The gummed flap is moistened to seal.
#9 Window Envelopes
A window #9 has a die-cut opening on the front that allows the recipient's address printed on an insert to show through. This eliminates the need to address the reply envelope separately. Window #9 envelopes are popular for remittance mailings and billing inserts where the address is already printed on the return stub.
Browse window envelopes to see available sizes and window configurations.
Security Tinted #9 Envelopes
Security tinted envelopes have a printed interior pattern that makes the contents unreadable when held up to light. This is important for checks, account numbers, personal information, or any document where content privacy matters. The tinting does not affect how the envelope looks from the outside.
Self Seal #9 Envelopes
Peel and seal #9 envelopes have a self-adhesive strip under a peel-off liner. The user peels and presses to seal without moisture. This is faster in high-volume operations and avoids the inconsistency of hand-licking gummed flaps. See the full range of self seal envelopes for options across sizes.
Colored #9 Envelopes
Colored and starburst #9 envelopes stand out in a stack of mail. They are used in direct mail campaigns, holiday appeals, and any mailing where differentiation matters. Starburst patterns add visual energy to donation response packages and promotional reply pieces. Explore colored envelopes for available color options.
How Does Postage Work for a #9 Envelope?
Postage for a #9 envelope depends on how it is being used. There are two scenarios: enclosed inside a #10, or mailed on its own.
When Enclosed Inside a #10 Envelope
No separate postage is needed on the #9 itself. The outer #10 envelope carries the full mailing under the sender's stamp or metered rate. The #9 simply rides along as part of the contents. The total weight of the outer mailing (envelope plus inserts plus the enclosed #9) determines what postage the sender pays on the #10.
A typical mailing containing one 8.5 x 11 sheet folded in thirds plus one blank #9 envelope stays well under 1 oz, so it mails at the base First-Class letter rate.
When Mailed on Its Own
A #9 envelope mailed as a standalone piece qualifies as a USPS First-Class letter as long as the contents bring the total weight under 3.5 oz and the piece meets the standard aspect ratio and thickness requirements. It goes through automation sorting normally without a non-machinable surcharge.
For current First-Class postage rates and a full breakdown of what affects letter-rate eligibility, see our #11 envelope postage guide for context on standard letter mailing rules.
Business Reply Mail (BRM)
A #9 envelope used as a Business Reply Mail piece lets recipients return it without paying postage. The sender applies for a BRM permit, prints the required indicia on the envelope, and pays the USPS a per-piece fee plus a permit fee only for the envelopes that are actually returned. This is common in nonprofit fundraising, subscription services, and financial billing where higher response rates justify the permit cost.
| Scenario | Who Pays Postage | When It Applies |
|---|---|---|
| Enclosed in #10, no BRM | Sender pays postage on #10 only | Standard reply envelopes |
| Mailed alone (pre-stamped) | Sender stamps the #9 in advance | High-touch or VIP mailings |
| Business Reply Mail | Sender pays per returned piece + permit fee | Donation campaigns, billing |
| Recipient-stamped reply | Recipient applies their own stamp | General reply envelopes |
How to Choose the Right #9 Envelope for Your Mailing
The right #9 envelope depends on three factors: what you are enclosing, who the recipient is, and whether you need privacy or visual differentiation.
Use this guide to narrow down your choice:
- Sending checks, account numbers, or personal data: Choose a security tinted #9. The interior pattern keeps contents private when held up to light.
- Remittance or billing with a return address on the stub: Choose a window #9 so the address shows through automatically without separate labeling.
- High-volume sealing operation: Choose a peel and seal version to speed up fulfillment and avoid moisture-activated gum.
- Nonprofit donation campaigns or seasonal direct mail: Choose colored or starburst #9 envelopes to stand out in the mailbox.
- Standard professional reply mail: A plain white wove gummed #9 keeps it clean and business-appropriate.
If you are also choosing the outer envelope, pairing your #9 reply piece with a matching #10 envelope gives you a coordinated, professional mailing package.
Can #9 Envelopes Be Custom Printed?
Yes. Custom printed #9 envelopes are available with your company name, return address, logo, or tagline. Custom printing is useful when:
- You want to pre-address the reply envelope with your return address so recipients do not have to write it themselves
- You want to reinforce your brand on every piece of the mailing package
- You are running a BRM program that requires the USPS indicia printed in the correct format
At Business Envelopes, custom printed #9 envelopes come with a PDF proof before production begins so you can confirm the layout before any envelopes are printed. Production time after proof approval is typically 7 to 10 business days. Blank stock is also available if you prefer to address them in-house.
Free ground shipping is included on all orders site-wide, with no minimum order requirement.
If you have recently sent out a mailing with a SASE enclosed, you may find our guide on SASE envelopes useful for understanding the difference between a self-addressed stamped envelope and a standard business reply envelope.