Envelope Sizes: The Complete US Chart, Plus USPS Mailing Limits

The short answer

The most common US envelope sizes are:

• #10 envelope: 4⅛ × 9½ in. The standard business envelope. It holds a letter-size (8½ × 11 in) sheet folded in thirds.
• A7 envelope: 5¼ × 7¼ in. The standard size for 5 × 7 in invitations and greeting cards.
• A2 envelope: 4⅜ × 5¾ in. Fits 4¼ × 5½ in cards (a letter sheet folded into quarters), including many thank-you notes.
• 6 × 9 in envelope. Holds a letter sheet folded in half, booklets and statements.
• 9 × 12 in envelope. Holds a letter-size sheet flat, unfolded.
• 10 × 13 in envelope. Holds thicker stacks of letter-size documents or catalogs flat.

Quick answer: Use a #10 for folded letters, an A7 for 5 × 7 cards, an A2 for 4¼ × 5½ cards, a 6 × 9 for half-folded letter sheets, and a 9 × 12 or 10 × 13 to mail documents flat. To mail at letter prices, USPS requires envelopes between 3½ × 5 in and 6⅛ × 11½ in, no thicker than ¼ in.

How envelope sizes are named

US envelope names look random, but they follow a few families:

• Commercial (business) envelopes use numbers such as #6¾, #9 and #10. Higher numbers are generally larger. They open on the long side and are used for letters, bills and statements.
• Announcement envelopes use A-sizes (A2, A6, A7, A9…) and fit greeting cards and invitations. They often have a square flap.
• Catalog envelopes open on the short side (open end), such as the classic kraft 9 × 12 envelope. They have numbers like #1, #6 or #10½.
• Booklet envelopes open on the long side (open side) and are easier for machine inserting.
• Square envelopes (5 × 5, 6 × 6 and so on) look striking but cost more to mail (see below).

Outside North America, envelopes follow the ISO C and DL series, which are matched to A-series paper sizes (see the international chart below).

Commercial (business) envelope sizes

#10 window envelopes

A standard #10 window envelope has a window of about 1⅛ × 4½ in placed about ⅞ in from the left edge and ½ in from the bottom. If you print invoices or letters for window envelopes, align the recipient address block to that position on your letterhead template. Double-window envelopes also show the return address in a second window at the top left.

Announcement (A-style) envelope sizes

These are the sizes stationery and card designers use most:

Rule of thumb: an envelope should be about ¼ in larger than the card in each direction, so the card slides in easily but doesn't move around.

Catalog and booklet envelope sizes

International (ISO) envelope sizes

US Letter paper is slightly wider and shorter than A4, so a US letter folded in thirds fits a DL envelope, but only just, and an A4 sheet folded in thirds fits a #10 envelope with room to spare.

Which envelope fits your paper?

USPS size rules for envelopes

Envelope size decides whether your mail is a letter, a flat (large envelope) or a package, and that decides the postage. These standards come from the USPS Domestic Mail Manual.

Letters

• Minimum: 5 in long × 3½ in high × 0.007 in thick. Pieces longer than 6 in or higher than 4¼ in must be at least 0.009 in thick.
• Maximum: 11½ in long × 6⅛ in high × ¼ in thick.
• Weight: First-Class letters can weigh up to 3.5 oz. Heavier letter-size pieces pay flat prices.

Flats (large envelopes)

• Bigger than a letter in any dimension, but no more than 15 in long × 12 in high × ¾ in thick.
• Must be flexible and uniformly thick (bumps cannot vary thickness by more than ¼ in).
• First-Class Mail letters and flats can weigh up to 13 oz.

A 9 × 12 or 10 × 13 envelope is a flat. Anything rigid, lumpy, or thicker than ¾ in is charged as a package instead.

What makes a letter "nonmachinable"

A letter-size envelope that can't go through USPS sorting machines pays a nonmachinable surcharge. Under the Domestic Mail Manual, a letter is nonmachinable if it:

1. Has a length-to-height ratio below 1.3 or above 2.5. This is why square envelopes always cost more to mail.
2. Is wrapped in plastic or has a non-paper exterior (paper windows are fine).
3. Has clasps, strings, buttons or similar closures.
4. Has uneven thickness from items like pens, keys or coins.
5. Is too rigid to bend easily.
6. Is too thin for its size (under 0.009 in when longer than 6 in or higher than 4¼ in).
7. Has the delivery address running parallel to the shorter side.

Wedding invitation tip: layered invitations with ribbons, wax seals or pockets often become nonmachinable or even flats. Take one finished, sealed invitation to the post office and ask them to weigh and measure it before you buy stamps for the whole batch.

Envelope materials and closures

Paper

• 24 lb bond (about 90 gsm) white wove is standard for business envelopes.
• 28 lb kraft is common for catalog envelopes, for strength.
• Text and cover-weight papers in colors and textures are used for invitations.
• Recycled papers are widely available in every common size.

Closures

• Gummed (lick-and-stick): cheapest; moisten to seal.
• Peel-and-seal: a release strip over adhesive; fast and clean.
• Self-seal (latex): two latex strips stick when pressed together.
• Clasp: a metal clasp plus gummed flap, reusable but nonmachinable for letters.
• String-and-button: reusable for inter-office mail; nonmachinable.

Flap styles

Commercial flaps (pointed or curved) are standard for business mail. Square flaps look modern on invitations. Euro flaps (deep V-shape) and contour flaps are decorative choices for stationery.

Envelopes for business: which sizes to stock

A small business rarely needs more than four or five envelope sizes:

1. #10 (plain and window): invoices, letters, contracts and checks.
2. #9 reply envelopes: fit inside a #10 for return payments or forms.
3. A2 or A7: thank-you cards and customer notes. Match the size to your printed cards.
4. 9 × 12 catalog: documents that must stay flat, such as certificates, brochures and signed paperwork.
5. 10 × 13 catalog: thicker document sets and catalogs.

Buying in boxes of 500 or 1,000 brings the unit cost down sharply, and branded printing usually becomes cost-effective at a few hundred envelopes or more.

Envelopes vs mailers for products

Envelopes are designed for documents. When you ship products, a mailer is usually better:

• Poly mailers are lightweight, waterproof and cheap for soft goods like apparel.
• Padded or bubble mailers protect small items such as jewelry, cosmetics and accessories.
• Rigid or paperboard mailers keep prints, photos and documents flat.
• Corrugated mailer boxes protect fragile items and create a branded unboxing.

If a product is thicker than ¾ in, rigid or lumpy, it will be charged as a package anyway, so a small mailer box often costs no more to ship and arrives in much better shape.

AccessoriesCustom Poly MailersPrinted, tear-resistant mailer bags with self-seal strips, in recycled options. Box StylesMailer BoxesSelf-locking corrugated mailers printed inside and out. The go-to box for DTC brands and subscriptions. AccessoriesThank You Cards & InsertsPrinted thank-you cards, care cards and discount inserts that drive repeat orders. AccessoriesCustom Stickers & LabelsSeal stickers, product labels and roll labels in paper, vinyl, kraft and foil.

Printing custom envelopes

Branded envelopes make invoices, thank-you notes and direct mail look professional. Before you print:

• Keep the right side clear for postage and the address area clear for the delivery address and barcode. USPS needs a clear zone along the bottom of letter-size mail for its barcode.
• Use the return address area in the top left for your logo and address.
• Avoid dark backgrounds behind the address; scanners need contrast.
• Print the back flap for a design touch that doesn't interfere with mailing.
• Order envelopes and cards together so sizes match.

How to measure an envelope

Measure the envelope closed and flat, edge to edge, ignoring the flap. US charts usually list the short side first (a #10 is 4⅛ × 9½ in), but some suppliers list the long side first, so check the listing. To choose a size, measure the card or folded sheet you need to insert, then add about ¼ in in each direction. For thick inserts such as layered invitations, add a little more so the envelope closes without bulging.

Common envelope mistakes

1. Choosing an envelope that is too tight. Cards that need forcing tear envelope seams and bend corners. Keep the ¼ in allowance.
2. Using square envelopes for large mailings without budgeting the surcharge. Every square letter pays extra postage.
3. Forgetting the weight of heavy card stock. A thick invitation plus inserts can pass 1 oz quickly. Weigh a finished sample.
4. Printing dark envelopes. Dark backgrounds make addresses hard to read and can slow automated sorting. Use a light label or address panel.
5. Ignoring the return-address area. Undeliverable mail needs a return address in the top left.
6. Mixing paper sizes and envelope systems. A4 in a #10 works, but letter-size paper in a DL envelope is tight. Match the system to the paper.

Envelope sizes by occasion

Envelope paper weights explained

Envelope paper is usually described in US bond or text pounds. A few common grades:

• 20 lb bond (about 75 gsm): lightweight, economical envelopes for bulk mailings.
• 24 lb bond (about 90 gsm): the everyday standard for business envelopes; opaque enough to hide most contents.
• 28 lb kraft (about 105 gsm): sturdy catalog envelopes that survive the mail with documents inside.
• 70–80 lb text (about 104–118 gsm): invitation and greeting card envelopes with a premium, substantial feel.

Security tint (a printed pattern inside the envelope) hides checks and personal information without needing heavier paper. See our paper weight chart to compare grades in GSM.

Printing on envelopes at home or in the office

Most desktop printers can print envelopes through a rear or manual feed:

1. Choose the envelope size in the print dialog (for example "Envelope #10" or "A7").
2. Check the orientation using the small diagram on the printer's tray, usually flap down and to one side.
3. Use smooth, flat envelopes. Envelopes with heavy textures, windows or clasps jam easily.
4. Print one test envelope before running a batch.
5. Let laser-printed envelopes cool before stacking, since the fuser can soften some adhesives.

For larger runs, branded envelopes printed by a commercial printer cost less per piece and look sharper.

Choosing envelopes for invitations, step by step

1. Finalize the card size first. 5 × 7 in pairs with an A7 envelope; 4¼ × 5½ in with an A2.
2. Decide on inner envelopes. Formal invitations sometimes use an inner envelope inside the outer one; the outer must then be one size up.
3. Choose paper weight and color to match the card. A heavier envelope (about 70–80 lb text) feels more premium.
4. Choose the flap. Square or euro flaps look elegant; pointed flaps look traditional.
5. Test a full set. Assemble one invitation with all inserts, seal it and weigh and measure it at the post office.
6. Order spares. Plan about 10% extra for addressing mistakes.

A short history of the envelope

For centuries, letters were simply folded and sealed with wax, and postage was usually charged by the number of sheets, so a separate envelope cost the recipient more. That changed in 1840, when the UK introduced uniform prepaid penny postage, charged by weight instead of sheets. Envelopes quickly became popular, and the first envelope-folding machines appeared in England in the mid-1840s. Today's standardized sizes grew out of the needs of printers, office machines and, above all, postal sorting equipment, which is why USPS size limits still shape the envelopes we buy.

Sources and further reading

• USPS Domestic Mail Manual 101: Physical standards for commercial letters, flats and parcels (Postal Explorer): the official size, thickness and machinability rules quoted in this guide.

Related products & pages

AccessoriesCustom Poly MailersPrinted, tear-resistant mailer bags with self-seal strips, in recycled options. AccessoriesThank You Cards & InsertsPrinted thank-you cards, care cards and discount inserts that drive repeat orders. Box StylesMailer BoxesSelf-locking corrugated mailers printed inside and out. The go-to box for DTC brands and subscriptions. AccessoriesCustom Stickers & LabelsSeal stickers, product labels and roll labels in paper, vinyl, kraft and foil.