LetterAgent
Mail physical letters by just asking your AI assistant.
Home > Guides > Envelope sizes
Use a #10 envelope (4-1/8 by 9-1/2 inches) for business letters: it's the size everything expects, from tri-folded letters to window envelopes. A #9 fits inside a #10 as a reply envelope, and an A2 (4-3/8 by 5-3/4) suits short notes or quarter-folded letters. Anything thick or stiff belongs in a flat mailer.
| Size | Dimensions | Best for |
|---|---|---|
| #10 | 4-1/8 x 9-1/2 in | The default business envelope; holds a tri-folded letter-size sheet |
| #9 | 3-7/8 x 8-7/8 in | Reply envelopes; sized to slip inside a #10 |
| #6-3/4 | 3-5/8 x 6-1/2 in | Personal notes and small enclosures |
| A2 | 4-3/8 x 5-3/4 in | Quarter-folded letters, RSVP and thank-you cards |
| A7 | 5-1/4 x 7-1/4 in | Half-folded letters and 5x7 invitations |
| 9x12 | 9 x 12 in | Unfolded letter-size pages, certificates, thick stacks |
The numbering looks random, but there is a system: the #9 was designed to fit inside the #10, which is why return envelopes in business mail are almost always #9s. If you are ordering envelopes for an office, #10 covers nearly everything, and a box of 9x12s covers the rest.
A standard letter-size sheet (8.5x11) tri-folds into a #10. Legal-size paper (8.5x14) is longer, so it folds in half to fit the same #10 envelope, which is why it is called the legal fold. Folding legal paper in thirds makes it too thick at the folds and it will not sit right in the envelope. If you are folding at all, the #10 is your envelope either way.
When the contents cannot fold or should not fold: certificates, photo prints, thick stacks of paper, anything with a wax seal or a clasp. A 9x12 rigid mailer keeps pages flat and doubles as a "do not bend" package. The postage runs higher than a letter stamp because oversized pieces sort as flats, but that is cheaper than reprinting a creased certificate. Anything rigid or lumpy inside also avoids the nonmachinable surcharge problem that plagues thick envelopes.
Window envelopes are #10s with a cutout positioned so a tri-folded letter's address panel shows through. They save you from addressing the envelope by hand, which is why businesses love them. The catch is the fold has to be exact: if the address panel lands even half an inch off, the window shows blank paper and the envelope goes nowhere. Fold carefully, or print the address on the envelope itself and skip the window.
Ready to mail it? Send your letter with LetterAgent - describe it to your AI assistant, approve the exact quoted price, and it's printed and mailed.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant. It can check a job's status any time. If it ever can't handle something: hello@getletteragent.com