The #10 envelope dominates business mail for good reason. It measures 4 1/8" x 9 1/2" (104.8 x 241.3 mm) and perfectly fits a standard 8.5" x 11" sheet folded into thirds - no trimming, no awkward bulges, no postage surprises. It’s the go-to for invoices, letters, and day-to-day correspondence.
Of course, not everything in business folds up that neatly.
When you're sending thicker packets, unfolded documents, or anything custom-sized, the #10 won’t cut it. That’s when the other business envelope sizes come into play.
Common Business Envelope Sizes: Comparison Table
Here’s a side-by-side comparison of business envelope sizes, what they hold, and when to use each one:
| Envelope Type | Size (inches) | Size (mm) | Paper Size It Holds | Common Use | Mails As |
|---|---|---|---|---|---|
| #6¾ | 3.625 x 6.5 | 92 x 165 | Personal check (3" x 6") | Small inserts, payments | Letter |
| #8⅝ | 3.625 x 8.625 | 92 x 219 | Slips, checks | Financial documents | Letter |
| #9 | 3.875 x 8.875 | 98 x 225 | Fits into #10 (1/3 of 8.5" x 11") | Reply mail, return envelopes | Letter |
| #10 | 4.125 x 9.5 | 105 x 241 | 8.5" x 11" tri-folded | Standard business correspondence | Letter |
| #11 | 4.5 x 10.375 | 114 x 264 | Multiple folded sheets | Promotional or legal mailings | Letter |
| #12 | 4.75 x 11 | 121 x 279 | Legal size tri-folded or thick contents | Multi-page statements, contracts | Letter |
| #14 | 5 x 11.5 | 127 x 292 | Bulky or unfolded legal docs | Thick contents, flat mailing | Letter / Flat |
Types of Business Envelopes and Sizes
Envelopes are built for specific jobs - some for folded letters, others for flat documents, cards, or bulky inserts. Using the wrong type can mean bad fits, bent edges, or unexpected postage costs.
This section breaks them into four practical categories: commercial, catalog/booklet, announcement, and specialty. For each, you'll get the key dimensions, what they’re made to hold, and what to watch out for when mailing.
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A. Commercial / Business Envelopes
These are the default envelopes for business mail - used daily in offices, billing departments, and fulfillment operations. They’re built to handle standard 8.5" x 11" documents and optimized for folding, printing, and postage efficiency.
| Envelope Type | Size (inches) | Size (mm) | Paper Size It Holds | Common Use | Mails As |
|---|---|---|---|---|---|
| #6¾ | 3.625 x 6.5 | 92 x 165 | Personal check (3" x 6") | Small inserts, payments | Letter |
| #8⅝ | 3.625 x 8.625 | 92 x 219 | Slips, checks | Financial documents | Letter |
| #9 | 3.875 x 8.875 | 98 x 225 | Fits into #10 (1/3 of 8.5" x 11") | Reply mail, return envelopes | Letter |
| #10 | 4.125 x 9.5 | 105 x 241 | 8.5" x 11" tri-folded | Standard business correspondence | Letter |
| #11 | 4.5 x 10.375 | 114 x 264 | Multiple folded sheets | Promotional or legal mailings | Letter |
| #12 | 4.75 x 11 | 121 x 279 | Legal size tri-folded | Multi-page statements, contracts | Letter |
| #14 | 5 x 11.5 | 127 x 292 | Bulky or unfolded legal docs | Thick contents, flat mailing | Letter / Flat |
What fits:
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#10: Holds a tri-folded 8.5" x 11" sheet - the standard for invoices and formal letters
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#9: Fits inside a #10 - commonly used as a return envelope
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#6¾: Ideal for payments or personal checks
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#11: Offers extra room for thicker or multi-page inserts
Mailing guidance:
To qualify for standard letter postage:
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Stay under 1 oz
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Use rectangular shapes
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Keep envelopes flexible and machinable
Envelopes that are rigid, unusually shaped, or oversized may trigger non-machinable surcharges or be bumped into higher USPS rate categories.
B. Catalog and Booklet Envelopes
These larger envelopes are designed for flat mailing - ideal when folding isn’t an option. They’re commonly used for contracts, reports, brochures, or other documents that need to arrive crisp and uncreased.
| Envelope Size | Dimensions (inches) | What It Holds | Mailing Guidance |
|---|---|---|---|
| 6" x 9" | 6 x 9 | Half-folded sheets or compact booklets | Always considered a flat by USPS |
| 9" x 12" | 9 x 12 | Full 8.5" x 11" sheets flat | Considered a flat by USPS |
| 10" x 13" | 10 x 13 | Thicker packets, catalogs, presentations | Flat rate if under 3/4" thick & flexible |
What Fits
Use these when content must stay flat and professional - especially legal docs, client reports, or anything too thick for a standard #10.
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6" x 9": Holds half-folded sheets or compact booklets
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9" x 12": Fits a full 8.5" x 11" sheet flat - perfect for contracts or multi-page documents
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10" x 13": Designed for thicker packets, catalogs, or presentation materials
Mailing Guidance
USPS classifies anything larger than 6 1/8" x 11 1/2" as a “large envelope” (flat) instead of a letter. That means:
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Higher postage rates
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Stricter thickness and flexibility rules
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Contents must be under 3/4" thick and flexible to qualify as machinable
Oversized or rigid mailings may be treated as parcels.
C. Announcement / Invitation Envelopes
These envelopes are designed to make an impression. A-style and square envelopes are commonly used for thank-you notes, branded inserts, and formal invitations where presentation matters as much as the message.
| Envelope Type | Dimensions (inches) | Fits | Mailing Notes |
|---|---|---|---|
| A2 | 4.375 x 5.75 | 4.25" x 5.5" cards | Standard letter if weight/size OK |
| A6 | 4.75 x 6.5 | 4" x 6" photos/cards | Standard letter if within limits |
| A7 | 5.25 x 7.25 | 5" x 7" inserts | Often used for formal invites |
| Square (e.g., 6.5" x 6.5") | 6.5 x 6.5 | Square cards | Non-machinable surcharge applies |
Note: A-style envelopes must still meet USPS requirements for flexibility, uniform thickness, and aspect ratio to avoid non-machinable charges.
What Fits
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A2 (4.375" x 5.75"): Fits 4.25" x 5.5" cards - great for thank-you notes or RSVP cards
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A6 (4.75" x 6.5"): Fits 4" x 6" photos or flat cards - often used for invites and holiday cards
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A7 (5.25" x 7.25"): Fits 5" x 7" inserts - a standard for formal announcements and marketing mail
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Square Envelopes (e.g., 6.5" x 6.5"): Designed for square cards - strong visual impact, but stricter mailing rules
Mailing Guidance
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Square envelopes are considered non-machinable by USPS, triggering a surcharge - even under 1 oz
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A-style envelopes (A2, A6, A7) usually qualify for standard letter postage if kept within size and weight limits
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For cost-sensitive mailings, stick with rectangular formats to avoid extra postage
Special Use Envelopes: When Function Overrides Size
Some business mail requires more than just the right dimensions - especially when security, branding, or protection are key.
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Windowed envelopes: Let pre-printed addresses show through; must be aligned precisely.
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Security-tinted envelopes: Hide sensitive info; ideal for billing and financial documents.
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Padded / bubble mailers: Protect small items in transit, but count as packages - not letters or flats.
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Custom-printed or colored envelopes: Great for branding, but must remain USPS-readable (avoid dark/glossy finishes).
These envelope styles follow standard USPS size rules if they’re flexible and machine-readable - otherwise, they may be bumped into parcel pricing.
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How to Choose the Right Business Envelope
Trying to figure out envelope sizes can feel like overkill - especially when all you want is to send something that looks clean and arrives without issues.
Envelope sizes can feel unnecessarily complicated - especially when you're just trying to send something simple and professional. But it really comes down to three things: what you're sending, how it should arrive, and what it’ll cost to mail.
Use these quick rules to simplify your decision:
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For everyday mail: Stick with the #10. It handles most letters, prints easily, and qualifies for standard postage.
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For documents that can’t be folded: Use a flat envelope like 9x12 or 10x13. It keeps contents crisp and professional.
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For design-forward mailings: Go with A-style or square envelopes for visual impact - just know square formats often cost more to send.
Match the envelope to the content, and your mail will do its job - clearly, professionally, and without surprises.
