Envelope sizes chart: No. 10, a7, 6 × 9 and more explained

The main envelope sizes at a glance

US envelopes fall into three families. Business envelopes, named with numbers, are long and narrow for folded letters. Announcement envelopes, named with an A, are proportioned for cards and invitations. Catalog and booklet envelopes, named by their inch dimensions, hold flat documents.

If you print a single envelope style for your business, make it the No. 10. It handles invoices, letters, contracts and most mailers. Add an A7 if you send thank you notes or invitations, and a 9 × 12 in if you ever send documents that should not be folded.

How to pick the right size for any insert

The working rule is to leave about 0.125 to 0.25 in of room on each dimension. Less than that and the insert jams or tears the envelope; much more and it slides around and looks cheap. Measure the insert, add the margin, and pick the nearest standard size.

For example, a 5 × 7 in card plus 0.25 in on each dimension is 5.25 × 7.25 in, which is exactly an A7. A 4.25 × 5.5 in card fits an A2. A half letter sheet at 5.5 × 8.5 in fits an A9. A letter sheet folded in thirds measures about 3.7 × 8.5 in and fits a No. 10 with room to spare, while a letter folded in half fits a 6 × 9 in.

Thicker inserts need more room. A card printed on heavy stock with a folded note inside, or a small booklet, may need the next size up. Always test with a real printed insert, not a sheet of copy paper.

USPS rules that affect postage

Envelope size has a direct effect on postage. USPS treats a piece as a letter if it is between 3.5 × 5 in and 6.125 × 11.5 in, no more than 0.25 in thick, and within the weight limit. Larger flexible pieces up to 12 × 15 in and 0.75 in thick are flats, which cost more.

Shape matters too. The length divided by the height must fall between 1.3 and 2.5 for a letter to be machinable. Square envelopes fall outside that range and are charged a nonmachinable surcharge, as are rigid or lumpy pieces.

If you love a square invitation, budget for the surcharge or pick an A7 or A9 for the same effect in a rectangular format. When in doubt, take a sealed sample to the post office before ordering a large quantity.

International sizes

Most other countries use ISO envelope sizes that pair with A series paper. DL at 110 × 220 mm is the international business envelope and holds A4 folded in thirds. C5 at 162 × 229 mm holds A4 folded in half, C6 at 114 × 162 mm holds A4 folded twice, and C4 at 229 × 324 mm holds A4 flat.

US letter paper folded in thirds fits a DL envelope, and A4 folded in thirds fits a No. 10, so mixed international correspondence usually works. The mismatch shows up with flat documents, since A4 is taller than US letter and can be tight in some 9 × 12 in envelopes.

Styles, flaps and windows

Beyond size, envelopes vary in a few ways that affect how they look and how you use them.

• Commercial style: opens on the long side with a pointed or square flap. Standard for business letters.
• Booklet style: opens on the long side with a wide flap. Easy to stuff by hand or machine.
• Catalog style: opens on the short end. Holds documents securely and suits heavier contents.
• Window envelopes: a clear panel shows the address printed on the letter, saving time for invoices and statements. Window positions are not standardized internationally, so align your letter template to the envelope you actually buy.
• Seal type: gummed flaps are economical; peel and seal strips are faster and cleaner for small batches.

Printing on envelopes

A printed return address turns a plain envelope into part of your brand. Keep it compact in the top left corner, well clear of the centered delivery address area and the postage corner at the top right, so it never confuses sorting equipment. Use the same logo and type as your letterhead for a matched set.

Color helps envelopes stand out in a stack of mail, which is why many small businesses use tinted envelopes for thank you notes and invitations. Keep the delivery address area light enough for strong contrast with dark ink so sorting machines can read it. For a full coordinated look, pair colored envelopes with thank you cards or note cards in matching tones.

Get the free print size cheat sheets

The free print size cheat sheets collect trim, bleed and safe area for cards, postcards, flyers and more, so you can match each printed piece to the envelope it will travel in.

Envelope sizing checklist

• Measure the final printed insert, including any folds.
• Add 0.125 to 0.25 in on each dimension and pick the nearest standard size.
• Keep letters within 6.125 × 11.5 in and 0.25 in thick for letter postage.
• Check the aspect ratio falls between 1.3 and 2.5 to avoid surcharges.
• Use No. 10 for business mail and A7 for cards and invitations.
• Keep the return address top left and the delivery area high contrast.
• Test a sealed sample at the post office before large mailings.