Business Envelope Sizes: Complete Comparison
Compare every standard business envelope size — #10, #9, A-series, 6×9, 9×12, and legal. Choose the right envelope by use case, postage class, and document fit.
| Envelope Specifications | |
|---|---|
| Dimensions (inches) | Various — see comparison |
| Dimensions (mm) | Various — see comparison |
| Weight Limit | 3.5 |
| USPS Postage Class | Varies by size |
| Primary Uses |
|
Choosing a business envelope comes down to four questions: what's going inside, whether the piece needs letter rate or flat rate, whether the recipient should see the address through a window, and how formal the format needs to be.
This page is a comparison reference covering every standard business envelope size, organized by use case rather than by number. For deep-dive specs on any individual size, follow the link to the dedicated page for that envelope.
Choosing a business envelope by use case
Standard letter mail (folded 8.5×11 documents)
- #10 envelope — 4.125" × 9.5" — the default for almost all business correspondence, invoices, and statements
- #9 envelope — 3.875" × 8.875" — the standard reply envelope inside a #10 outer (donation appeals, billing replies)
Invitations and announcements
- A6 envelope — 4.75" × 6.5" — save-the-dates, shower invitations, photo announcements
- A7 envelope — 5.25" × 7.25" — wedding invitations (the dominant choice), formal announcements, holiday photo cards
- A9 envelope — 5.75" × 8.75" — larger formal invitations, programs, half-page brochures
- A10 envelope — 6" × 9.5" — oversized announcements, premium direct mail (note: just under letter-rate maximum)
Legal-size document mailings
- #14 legal envelope — 5" × 11.5" — fits 8.5 × 14 paper folded in half
- #12 legal envelope — 4.75" × 11" — fits 8.5 × 14 paper folded in thirds
- #11 legal envelope — 4.5" × 10.375" — smaller variant of trifolded legal
Booklets and multi-page documents
- 6×9 envelope — booklets, brochures, multi-page direct mail (often reclassified as flat rate)
Unfolded letter-size and larger documents
- 9×12 envelope — unfolded 8.5 × 11 documents, certificates, photography prints, annual reports — always flat rate
Window envelopes
- Window envelopes available in #9 and #10 (most common), with single-window and double-window varieties for billing, statements, and pre-addressed contents
Decision tree
Start with the document. If it's an 8.5 × 11 letter folded in thirds, use a #10. If it's a 5 × 7 invitation, use an A7. If it's an unfolded 8.5 × 11 document, use a 9×12 flat. If it's an 8.5 × 14 legal document, use a #14. For everything else, check the comparison table above and consider the postage class first — letter rate at $0.78 versus flat rate at $1.63 is a meaningful cost difference at any volume.
Postage classes by envelope type
Letter rate ($0.78 starting in 2026)
All envelopes within USPS letter-size limits — 6.125" × 11.5" × 0.25", under 3.5 ounces, aspect ratio 1.3–2.5:
- #10, #9, all A-series (A6, A7, A9, A10)
- Legal envelopes (#11, #12, #14)
- 6×9 envelopes when contents are thin enough
Flat rate ($1.63 starting in 2026)
Envelopes that exceed letter-size limits or contain bulky enough contents to be reclassified:
- 9×12 (always flat — exceeds letter dimensions)
- 10×13 catalog envelopes
- 6×9 envelopes with thick booklet or brochure inserts
- Any letter-size envelope reclassified due to thickness, weight, or rigidity
Nonmachinable surcharge ($0.44 in 2026)
Letter-size envelopes that fail machinability standards — square envelopes, pieces with rigid contents, very thick or uneven pieces — are charged letter rate plus a $0.44 surcharge per piece.
Volume discounts
Bulk presort and automation discounts can reduce per-piece costs by $0.10–$0.30 below retail rates for letter-size pieces, and by larger amounts for flats. For mailings of 200 pieces or more, working with a print and mail partner that handles presort is the only way to capture these savings consistently. Cornerstone direct mail handles presort, automation, and full BMEU induction on every qualifying job.
Last verified: May 2026. Source: USPS Domestic Mail Manual 201, 301, and current rate schedules.
Sean is a USPS-certified Mailpiece Design Professional (MDP) with 25+ years of experience producing compliant direct mail campaigns for Hudson Valley businesses. He has processed over 2.3 million mail pieces through the USPS Business Mail Entry Unit in New Paltz, NY since 1998.
Frequently Asked Questions
What is the most common business envelope size?
The #10 envelope (4.125" × 9.5") is by far the most common business envelope in the United States. It is the default for invoices, statements, business letters, and most direct mail.
Which business envelope is cheapest to mail?
Any letter-size envelope mails at the same First-Class letter rate of $0.78 in 2026 — #9, #10, A-series, and legal envelopes all share this rate. The cheapest practical option for personal-feeling mail is the #10 because it is widely stocked and machinable.
What's the difference between a letter envelope and a flat envelope?
Letter envelopes are 6.125" × 11.5" × 0.25" or smaller, weigh under 3.5 ounces, and have an aspect ratio between 1.3 and 2.5. They mail at $0.78 starting. Flat envelopes (large envelopes) exceed letter-size limits, can be up to 12" × 15" × 0.75", weigh up to 13 ounces, and start at $1.63 postage.
When should I use a 9×12 envelope instead of a #10?
Use a 9×12 when documents must arrive unfolded — certificates, contracts, photography prints, and important presentations. Use a #10 when contents can be folded and the lower postage cost ($0.78 vs $1.63) matters.
What size envelope do I need for wedding invitations?
For 5 by 7 inch wedding invitations (the standard size), use an A7 envelope (5.25" × 7.25"). This is the dominant wedding invitation envelope in the US.
Where can I order custom business envelopes?
Cornerstone Services prints custom envelopes in every standard size with your branding, return address, window placement, and security tints — and pairs envelope production with direct mail and printing services. Get a quote.
Not sure this is the right size?
Compare all standard envelope sizes side-by-side, then call us. We help Hudson Valley businesses choose, print, and mail the right format every day.
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