The short answer
The standard US business envelope is the No. 10, which measures 4⅛ × 9½ inches and holds an 8½ × 11 letter folded in thirds. Cards and invitations use A-style envelopes such as the A2 (4⅜ × 5¾), A7 (5¼ × 7¼), and A9 (5¾ × 8¾), and unfolded letter paper goes in a 9 × 12 catalog or booklet envelope. For USPS, a rectangular envelope from 5 × 3.5 to 11.5 × 6.125 inches and up to 1/4 inch thick mails as a letter; bigger envelopes pay large-envelope (flat) prices.
What are envelope sizes called?
US envelope sizes fall into a few families, each named in its own way. Business, or commercial, envelopes have numbers such as No. 10. According to the Envelope Manufacturers Association, the numbers come from a size list William Irwin Martin prepared for the Stationers’ Handbook of 1876, based on the cutting dies of the time, which is why a No. 10 measures nothing like 10 inches.
Announcement envelopes, also called A-style, carry an A and a number, such as A7, and have a square flap. Baronial envelopes have a long pointed flap and are used mainly for cards and formal invitations; some are named by bar size, and a 4 Bar baronial has the same 3⅝ × 5⅛ inch dimensions as an A1. Larger envelopes are named by their dimensions: booklet envelopes open on the long side, while catalog envelopes open on the short end. The manufacturer charts cited here list the short side first, so a No. 10 is written 4⅛ × 9½.
Business envelope sizes: No. 6¾ to No. 14
Tension, an envelope manufacturer, says the No. 10 is the most frequently produced business envelope, followed by the No. 9, and the most common outer envelope for bills and statements. The No. 9 is typically a reply envelope sent inside a No. 10, and the Envelope Manufacturers Association notes that the No. 6¾ is often used as a remittance envelope for payments. All of these sizes mail as letters when they stay within USPS limits.
| Size | Envelope (inches) | Largest insert (inches) |
|---|---|---|
| No. 6¾ | 3⅝ × 6½ | 3⅜ × 6¼ |
| No. 9 | 3⅞ × 8⅞ | 3⅝ × 8⅝ |
| No. 10 | 4⅛ × 9½ | 3⅞ × 9¼ |
| No. 11 | 4½ × 10⅜ | 4¼ × 10⅛ |
| No. 12 | 4¾ × 11 | 4½ × 10¾ |
| No. 14 | 5 × 11½ | 4¾ × 11¼ |
What is the size of a 9 × 4 envelope?
No standard business envelope measures exactly 4 × 9 inches. The two closest standard sizes are the No. 10, at 4⅛ × 9½ inches, and the No. 9, at 3⅞ × 8⅞ inches. Check the exact measurements before you buy if one envelope must fit inside another or a particular insert must fit inside it: a No. 9 is made to travel inside a No. 10, but a letter folded in thirds, about 3⅔ inches tall, is slightly taller than the 3⅝-inch largest insert Tension lists for a No. 9.
A-style envelope sizes: A1, A2, A6, A7, and A9
A-style envelopes are sized to fit common card sizes. An A7 envelope, for example, measures 5¼ × 7¼ inches and fits a 5 × 7 card. Insert sizes vary slightly by maker: Leader Paper Products lists an A1 for inserts up to 3½ × 4⅞ inches, while Western States Envelope & Label lists 3½ × 5 inches.
| Size | Envelope (inches) | Largest insert (inches) | Fits |
|---|---|---|---|
| A1 | 3⅝ × 5⅛ | 3½ × 4⅞ | Small cards about 3½ × 5 |
| A2 | 4⅜ × 5¾ | 4¼ × 5½ | A half sheet of letter paper folded in half |
| A6 | 4¾ × 6½ | 4⅝ × 6¼ | 4 × 6 photos and cards |
| A7 | 5¼ × 7¼ | 5 × 7 | 5 × 7 cards and invitations |
| A9 | 5¾ × 8¾ | 5⅝ × 8⅝ | A full letter sheet folded in half |
Catalog and booklet envelopes: 6 × 9, 9 × 12, and 10 × 13
Use these when the contents shouldn’t be folded or folded only once. The 9 × 12 is the most popular booklet size and holds an unfolded 8½ × 11 sheet. The 6 × 9 is often used for letter paper folded in half; because it is within USPS letter dimensions, it can mail at the letter price if it stays no more than 1/4 inch thick and has no clasp.
| Size (inches) | Largest insert (inches) | Typical contents |
|---|---|---|
| 6 × 9 | 5¾ × 8¾ | 8½ × 11 sheets folded in half |
| 9 × 12 | 8¾ × 11¾ | Unfolded 8½ × 11 documents |
| 10 × 13 | 9¾ × 12¾ | Unfolded documents with more room |
Square envelope sizes and the extra postage
Square envelopes come in sizes such as 5 × 5, 5½ × 5½, 6 × 6, 6½ × 6½, 7½ × 7½, 9 × 9, and 10 × 10 inches in Western States Envelope & Label’s chart. They cost more to mail. A square envelope up to 6⅛ inches on a side is a letter with a length-to-height ratio of 1, below USPS’s 1.3 minimum, so it pays the $0.49 nonmachinable surcharge: $1.31 for 1 ounce. A larger square, such as 6½ × 6½, is taller than the 6.125-inch letter limit and pays large-envelope prices, from $1.69.
Which sizes mail as letters, flats, or nonmachinable mail
USPS decides the price by shape. A letter is 5 to 11.5 inches long, 3.5 to 6.125 inches high, 0.007 to 0.25 inch thick, and up to 3.5 ounces. A large envelope, or flat, exceeds at least one letter maximum but is no more than 15 × 12 inches and 3/4 inch thick; it must bend, be rectangular, and be evenly thick, or it pays package prices. A letter is also nonmachinable if its length divided by its height is under 1.3 or over 2.5, if it has a clasp, string, or button, or if it is too rigid or lumpy.
The table applies those rules to envelopes up to 1 ounce that are flat and no more than 1/4 inch thick, at USPS retail prices effective July 12, 2026, unchanged in the October 4, 2026 Notice 123.
| Envelope | USPS shape | 1-ounce postage |
|---|---|---|
| No. 6¾, 9, 10, 11, 12, or 14 | Letter | $0.82 |
| A1, A2, A6, A7, or A9 | Letter | $0.82 |
| 6 × 9 booklet, no clasp | Letter | $0.82 |
| 5 × 5 to 6 × 6 square | Nonmachinable letter | $1.31 |
| 6½ × 6½ or larger square | Large envelope (flat) | $1.69 |
| 9 × 12 or 10 × 13 | Large envelope (flat) | $1.69 |
What size envelopes take a regular stamp?
One Forever stamp, $0.82, covers any rectangular letter-size envelope that weighs up to 1 ounce and has no surcharge features. That includes every business size from No. 6¾ to No. 14, the A-style sizes from A1 to A9, and a 6 × 9 without a clasp. The A2 is close to the limit: its 5¾ × 4⅜ inch shape has a ratio of about 1.31, just above the 1.3 minimum.
Two details can still add cost. Pieces longer than 6 inches or taller than 4¼ inches must be at least 0.009 inch thick, or they pay the nonmachinable surcharge. And weight counts: USPS says about four sheets of printer paper in a business-size envelope weigh 1 ounce, and heavier letters need $0.29 more for each added ounce up to 3.5 ounces. Postcards have their own price and size limits.
Envelope size when you mail with IRIS
IRIS prints and mails individual letters from an uploaded US Letter-size PDF, so you don’t choose or buy an envelope; IRIS prints the letter, puts it in an envelope, and mails it by USPS First-Class Mail to a US address. A one-page black-and-white letter costs $3.20. IRIS doesn’t mail large (flat) envelopes or enclose physical items, so use a 9 × 12 envelope at the Post Office if pages must arrive unfolded.
For postcards, IRIS offers four sizes with full-color printing and First-Class postage included: 4 × 6 and Small 5.5 × 4.25 for $3.49, and 6 × 9 and Pro 8.5 × 5.5 for $4.49.
Sources & further reading
Product details checked against the IRIS API contract when this article was published. Platform references can change; consult the linked documentation when implementing.
- Envelope Manufacturers Association: The EMA Guide to Envelopes & Mailing (PDF)
- Tension Envelope: Common Envelope Sizes
- Tension Envelope: #10 Envelope Size
- Tension Envelope: What Is a #9 Envelope?
- Leader Paper Products: Announcement Envelopes
- Leader Paper Products: Baronial Envelopes
- Leader Paper Products: Booklet and Catalog Envelopes
- Western States Envelope & Label: Envelope Resource Guide, June 2025 (PDF)
- USPS Domestic Mail Manual 101: Physical Standards
- USPS Publication 25: Designing Letter and Reply Mail (PDF)
- USPS Notice 123: Price List (Postal Explorer)
- USPS: How to Send a Letter or Postcard
- USPS Postal Explorer: Tips and Tools for Measuring Letters and Large Envelopes
Published by IRIS Postal Forms, operated by Great Lakes Computing Office LLC. Send a correction.