The standard business envelope is the #10, which measures 4-1/8 x 9-1/2 in and holds an 8.5 x 11 in letter folded in thirds. Cards go in the A-series: an A2 (4-3/8 x 5-3/4 in) for 4.25 x 5.5 in cards and an A7 (5-1/4 x 7-1/4 in) for 5 x 7 in cards. Unfolded letter pages go in a 9 x 12 in envelope, or a 10 x 13 in when they are inside a presentation folder.
Size also decides postage. Anything taller than 6-1/8 in, longer than 11-1/2 in or thicker than 1/4 in mails as a flat, at a higher price than a letter.
What are the standard envelope sizes?
| Envelope | Size (in) | What fits | Mails as |
|---|---|---|---|
| #6 3/4 | 3-5/8 x 6-1/2 | Checks, receipts, small reply cards | Letter |
| #9 | 3-7/8 x 8-7/8 | A reply envelope that slides inside a #10 | Letter |
| #10 | 4-1/8 x 9-1/2 | 8.5 x 11 sheet folded in thirds | Letter |
| A2 | 4-3/8 x 5-3/4 | 4.25 x 5.5 cards and RSVP cards | Letter |
| A6 | 4-3/4 x 6-1/2 | Cards up to 4-5/8 x 6-1/4 | Letter |
| A7 | 5-1/4 x 7-1/4 | 5 x 7 cards and invitations | Letter |
| 6 x 9 | 6 x 9 | 8.5 x 11 folded in half, 5.5 x 8.5 booklets | Letter, if 1/4 in thick or less |
| 9 x 12 | 9 x 12 | Unfolded 8.5 x 11 sheets, certificates | Flat |
| 10 x 13 | 10 x 13 | A 9 x 12 folder with contents, thick reports | Flat |
Envelope dimensions are from Cenveo's Envelope Essentials guide, which lists A2, A6 and A7 under their older baronial names (5-1/2 Bar, 6 Bar and Lee). The mail class column applies the size limits in the USPS Domestic Mail Manual, section 101.
Large envelopes come in two styles. Booklet envelopes open on the long side, which suits machine inserting and leaves the back clear for printing. Catalog envelopes open on the short end and have a center seam, which holds heavier contents but is usually stuffed by hand or on specific equipment.
Which envelope fits a folded letter?
A letter folded in thirds goes in a #10. Folded once, it goes in a 6 x 9. Unfolded, it needs a 9 x 12. The #9 exists mostly as a return envelope: it is small enough to travel inside a #10 with a statement or invoice, already addressed back to you.
A common pairing for patient billing is a statement in a #10 window envelope with a #9 reply envelope tucked inside. Printing the practice letterhead and envelopes as one matched set keeps the logo and colors identical across correspondence and bills. The rest of a practice's print, from intake forms to signage, is in our medical practice printing guide.
Where does the window go on a #10 window envelope?
A common #10 window is 1-1/8 x 4-1/2 in, placed 7/8 in from the left edge and 1/2 in from the bottom. It is not universal: other makers and styles place the window differently, including right-side windows. Always lay out the letter against the envelope you will actually use.
A double window envelope adds a second window in the upper left for the return address printed on the letter itself, which is common for invoices and statements. Both windows have to line up with the insert after it is folded.
USPS sets the rules for what shows through the window in Publication 25, section 3-9:
- At least 1/8 in of clear space between the address block and every edge of the window, even when the insert slides to its limits inside the envelope.
- The address, and any barcode visible through the window, printed on white or very light paper.
- Window film clear and untinted, glued down on every edge.
Before approving a window envelope job, fold a printed proof of the letter or invoice, put it in a real envelope, and tap the envelope on each of its four edges. If any line of the address slips behind the paper, the post office will see the same thing. Most problems come from a letterhead designed on screen with the address a little too high or too long, or from a mail-merge that adds a fifth address line nobody planned for.
When does an envelope stop counting as a letter?
An envelope mails as a letter when it is no more than 6-1/8 in high, 11-1/2 in long and 1/4 in thick, and weighs no more than 3.5 oz. Go over any of those and it is a flat, which USPS also calls a large envelope, and it pays flat prices.
| Mail class | Minimum | Maximum | Watch for |
|---|---|---|---|
| Letter | 3-1/2 x 5 in, 0.007 in thick | 6-1/8 x 11-1/2 in, 1/4 in thick, 3.5 oz | Length divided by height must be 1.3 to 2.5, or a nonmachinable surcharge applies |
| Flat (large envelope) | Over a letter limit in any one dimension | 12 x 15 in, 3/4 in thick; First-Class up to 13 oz | Must bend and be uniformly thick, or it is priced as a parcel |
Two details catch people out. An A2 envelope has a ratio of about 1.31, just inside the machinable range, while any square envelope is 1.0 and pays the surcharge; our holiday card guide covers that cost. And a 6 x 9 envelope is still a letter, since 6 in is under the 6-1/8 in limit, until a thick booklet pushes it past 1/4 in.
Rigidity matters for flats too. The DMM says flat-size pieces must be flexible, and flats that fail that test are priced as parcels. A diploma or certificate mailed in a 9 x 12 envelope with a stiff board for protection may not bend enough to qualify, so weigh the protection against the postage.
What else belongs in a matching stationery set?
The envelope is one piece of a set, and the rest should be specified together so stock, color and logo match across all of it.
- Letterheads: 70lb or 80lb text, smooth or laid, printed in ink or foil, with margins kept inside laser-printer safe zones so your office printer does not smear the top inch.
- Window envelopes: single and double window, for statements and invoices that are printed and mailed from the office.
- Presentation folders: business card slits, reinforced spines and die-cut windows. A folder sized for letter pages mails in a 10 x 13 envelope.
- Practice letterhead and envelopes: matched stationery for medical, dental and therapy practices, for correspondence and billing.
- Report card envelopes: printed envelopes for report card distribution. Size the envelope to the card unfolded, so it is not creased in a backpack. More for schools in our school printing guide.
- Binder inserts and spines: printed covers and spines for ring binders, matched to the letterhead for proposals and manuals.
- Name tags and desk plates: reusable badges and engraved-look desk signage for reception and front desks.
- Certificates and awards: recognition certificates on premium stock. A letter-size certificate mails flat in a 9 x 12.
- Diplomas: printed on premium stock with optional foil seals.
Envelope size questions
What size is a standard business envelope?
The #10, at 4-1/8 x 9-1/2 inches. It holds an 8.5 x 11 inch letter folded in thirds and is the default for letters, invoices and statements.
What size envelope fits a 5 x 7 card?
An A7 envelope, which measures 5-1/4 x 7-1/4 inches. A 4.25 x 5.5 inch card takes an A2, at 4-3/8 x 5-3/4 inches.
What envelope holds 8.5 x 11 paper without folding?
A 9 x 12 inch envelope. Use a 10 x 13 inch envelope if the pages are in a presentation folder or the stack is thick.
Where is the window on a #10 window envelope?
On a standard #10 the window is usually 1-1/8 x 4-1/2 inches, placed 7/8 inch from the left and 1/2 inch from the bottom. Some brands differ, so check the envelope you will use. USPS requires at least 1/8 inch of clearance around the address inside the window.
Is a 6 x 9 envelope a letter or a flat?
A letter, as long as it is no more than 1/4 inch thick and weighs 3.5 ounces or less. Thicker or heavier, it mails as a flat.
See our letterhead and envelope printing range, or send the letter or form you want to mail and we will match the envelope and window to it. If those forms are multi-part, the carbonless forms guide explains the options.
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