The #6 3/4 is smaller than a #9, smaller than a #10, and serves a completely different purpose than either. Where the #9 is a standard return envelope and the #10 is the outgoing workhorse, the #6 3/4 is the response device. Its size is not a limitation. It is the point.
We carry the #6 3/4 in two distinct formats. The security tinted format serves payment and billing reply programs where the returned contents carry sensitive data. The remittance format serves nonprofit and donation programs where the large flap is the response form. This guide covers both completely, including dimensions, nesting math, what to print, how each format works, return mail options, and the programs that use each one.
What Is the #6 3/4 Envelope and What Are Its Exact Dimensions?
Core specifications
- Closed size: 3-5/8 x 6-1/2 inches (3.625 x 6.5 inches) | 92 x 165 mm
- Maximum enclosure: approximately 3-3/8 x 6-1/4 inches
- Paper weight: 24 lb white wove (standard)
- Postage (2026): USPS First-Class letter rate, $0.82 for the first ounce
- Aspect ratio: approximately 1.79 , within USPS machinable range of 1.3 to 2.5
- USPS classification: Letter rate , rectangular, uniformly thick, not rigid
- Compatible outgoing: Nests inside a #10 with 1/4-inch height and 2-3/4-inch length clearance
The "6 3/4" designation comes from William Irwin Martin's 1876 commercial envelope numbering system, where sizes between whole numbers received fractional designations. The 6 3/4 sits between the #6 and #7 in the original catalog sequence. It is one of the most consistently ordered reply and remittance envelope sizes in the US market, specifically because of its nesting relationship with the #10 outgoing envelope.
Why the #6 3/4 Fits Inside a #10 Outgoing Envelope
A #10 interior is approximately 3-7/8 x 9-1/4 inches. A #6 3/4 at 3-5/8 x 6-1/2 inches fits with approximately 1/4-inch clearance in height and 2-3/4-inch clearance in length. The height clearance is tight but workable: the #6 3/4 slides in without forcing. The length clearance is generous enough that a folded one- or two-page letter fits alongside it comfortably in the same #10 outgoing envelope.
Compare this with the #9 reply envelope (3-7/8 x 8-7/8 inches): the #9 fits in a #10 with essentially no height clearance and only about 3/8-inch length clearance. A full one-page letter folded alongside a #9 in a #10 is a tight fit that requires careful assembly and produces a bulky piece near the non-machinable thickness threshold.
The #6 3/4 inside a #10 provides meaningful working clearance for the return envelope and any enclosures alongside it. The assembled package consistently falls below the non-machinable surcharge threshold. This is the operational reason that donation programs and billing programs with multiple inserts choose the #6 3/4 over the #9 for their return envelope.
The Two #6 3/4 Formats: Security Tinted and Remittance
#6 3/4 security tinted for payment and billing reply programs
The security tinted #6 3/4 is the correct format for any program where the returned contents carry sensitive data. Healthcare billing programs sending statement packages: the patient returns a payment authorization form with credit card details. Financial services programs sending monthly statements: the customer returns a check with account information. Legal programs: a signed authorization or settlement agreement comes back with personal identification information.
In all of these cases, the contents of the returned envelope contain information that should not be readable through the envelope during postal return handling. The security tint interior, a dense printed crosshatch or geometric pattern on the full interior surface, blocks transmitted light through the paper, making any document inside unreadable when the envelope is held to a light source. This is exactly the same function the security tint serves on outgoing #10 envelopes, applied now to the return envelope for programs where the reply is as sensitive as the original mailing.
The security tinted #6 3/4 is available with a gummed flap as standard. It is also available with a peel-and-seal closure for programs where the recipient seals the envelope themselves and a moisture-free closure is preferred. See the full self-seal range at self seal envelopes.
For a complete guide to how security tinting works and when it is required, see security tinted envelopes guide.
#6 3/4 remittance for nonprofit donations and pledge programs
The remittance format is a different product from the standard security tinted #6 3/4. It shares the same outer dimensions (3-5/8 x 6-1/2 inches) but has an oversized flap, a larger-than-standard flap that folds over the sealed envelope face and carries a printed response form. The flap is the response device. Gift amount checkboxes, donor name and address fields, payment method options, and gift purpose lines are all printed on the large flap.
When the donor fills in the form and seals the envelope, the flap folds down to become the outward face of the sealed piece. The completed form and the payment are enclosed together inside. This design collapses the separate reply card into the envelope itself. Instead of a three-piece package (letter, reply card, return envelope), the program becomes a two-piece package: the appeal letter and the remittance envelope. The donor experience is simpler: fill in the flap, enclose payment, seal, stamp, mail.
For a complete guide to designing the flap, field layout, print specifications, and USPS compliance for the remittance format, see the 6 3/4 remittance envelope template guide.
What to Print on Each #6 3/4 Format
Security tinted #6 3/4 face (for billing and payment reply)
Organization name and return address in the upper left. Delivery address in the OCR zone. A "Please affix postage" instruction near the upper right if using Courtesy Reply Mail. Nothing in the barcode clear zone at the bottom 5/8 inch. For Business Reply Mail programs, the face must include the FIM mark, permit number, and Intelligent Mail barcode in USPS-compliant positions.
Remittance #6 3/4 large flap (for nonprofit and donation programs)
The flap carries gift amount options in checkboxes (typically three to four amounts plus a write-in box), donor name and address fields, payment method options, a brief gift purpose line, and an email field if collecting for receipt acknowledgment. Keep the flap concise: every additional field reduces completion rate. The flap is a transaction device, not a survey form. The organization name and return address on the envelope face (not the flap) identify where the sealed piece mails to.
Return Mail Options: Business Reply Mail vs Courtesy Reply Mail
Any #6 3/4 return envelope can be set up as a Courtesy Reply Mail (CRM) piece or a Business Reply Mail (BRM) piece. The choice affects who pays the return postage and how many pieces come back.
Courtesy Reply Mail (CRM)
A CRM envelope is pre-addressed with your organization's return address and pre-printed with the postal barcode, but the sender affixes their own stamp before dropping it in the mail. Your organization pays nothing on envelopes that are never returned. CRM is the lower-cost option per piece and requires no USPS permit beyond the standard printing requirements.
CRM is appropriate when the respondent is already motivated to reply, for example in utility billing programs where the customer intends to pay their bill regardless of who covers the postage. The presence of a pre-addressed, pre-barcoded envelope eliminates one friction point without adding per-piece postage cost.
Business Reply Mail (BRM)
A BRM envelope allows the sender to return it postage-free. Your organization pays the postage only on pieces actually returned, plus an annual USPS permit fee and a per-piece surcharge on top of the standard postage. BRM requires a USPS First-Class Mail BRM permit and must follow strict USPS formatting rules for the FIM mark, permit number, and Intelligent Mail barcode placement.
BRM typically generates higher response rates than CRM because removing the postage barrier reduces the effort required to respond. Fundraising campaigns commonly use BRM for this reason: even a small reduction in friction between the donor and the donation matters for response rate, especially with first-time or lapsed donors.
#6 3/4 vs #9: Choosing the Right Reply Format
| Factor | #6 3/4 | #9 |
|---|---|---|
| Dimensions | 3-5/8 x 6-1/2" | 3-7/8 x 8-7/8" |
| Fits in #10 alongside letter | Comfortably , 2-3/4" length clearance | Tightly , 3/8" length clearance |
| Remittance large-flap format | Yes , standard remittance format | Yes , also available in #9 |
| Security tinted format | Yes | Yes |
| BRM industry standard size | Less common | Yes , BRM programs typically specify #9 |
| Best for | Donation programs, multi-insert billing, nonprofit appeals | Standard billing reply, single-form return, BRM programs |
| Total package weight (with #10 outer and letter) | Typically under 1 oz , single-stamp rate | Near or slightly over 1 oz , potentially two stamps |
The #9 is better when the program sends only a single document alongside the return envelope. It is also the right choice when BRM design requirements specify #9, or when the reply card needs the extra length a #9 provides. The #6 3/4 is better when the outgoing package includes multiple inserts that need clearance in the #10. It is also the right choice for programs using the remittance flap format, or when total package weight is near the single-ounce threshold and a smaller return envelope keeps the assembly under the single-stamp rate.
Postage on the #6 3/4: What Goes Out, What Comes Back
The #6 3/4 is always enclosed inside an outgoing #10 (or occasionally a 6x9 for larger nonprofit packages). The outgoing postage is paid on the #10 outer at $0.82 for the first ounce in 2026. The #6 3/4 is what the recipient uses to return their response. They pay the return postage with a stamp unless the program uses BRM, in which case your organization pays only on pieces actually returned.
A typical outgoing package weighs roughly 12 to 14 grams. That includes the letter (approximately 5 grams), the #6 3/4 return envelope (3 to 4 grams), and the #10 outer envelope (4 to 5 grams). This falls well within the first-ounce bracket at approximately 28 grams. Adding a second insert (a response card or donation slip) adds approximately 2 to 3 grams, still comfortably within a single stamp for most single-sheet programs.
Custom Printing on #6 3/4 Envelopes
Custom printing is available on all #6 3/4 formats through custom printed envelopes. For security tinted formats, printing goes on the envelope face: organization name, return address, logo, and postal barcode if using CRM or BRM. The interior tinting is separate from the exterior print process and is not affected by printing on the face.
For the remittance format, printing covers both the envelope face (organization name and return address, which identifies where the sealed piece mails to) and the extended flap (the response form: giving tiers, donor fields, payment options). The flap artwork is typically provided by the ordering organization to match their donation program design. A PDF proof is generated before production begins. Artwork stays on file for reorders, so future orders pull the same specifications without re-uploading.
Custom printed orders on all #6 3/4 formats ship within 7 to 10 business days from proof approval. Blank stock ships same or next business day. No minimums. Free ground shipping on every order. See all available formats in the regular envelopes range, or browse the complete catalog at all items.
Why Choose Business Envelopes for #6 3/4 Programs
Business Envelopes has supplied #6 3/4 reply and remittance envelopes to nonprofits, healthcare billing offices, financial services organizations, schools, and churches across the United States since 1997. The security tinted stock uses full interior coverage that blocks read-through on the front face, where interception actually occurs. The remittance format uses a properly proportioned extended flap that provides adequate writing surface for standard donation form layouts without crowding the fields.
No minimums on any order. Free ground shipping on every order to the contiguous 48 states. Blank stock ships same or next business day. Custom-printed orders ship within 7 to 10 business days from proof approval. Artwork stays on file for reorders. PDF proof before every production run. For the complete outgoing program alongside your #6 3/4 return envelopes, browse #10 envelopes, colored envelopes for starburst and wove color outer options, and window envelopes for outgoing billing programs.