A10 envelope size and what fits in it

What the A10 envelope is for

The A10 is the longest of the US announcement envelopes, the stationery sizes such as A2, A7 and A9 that card makers and print shops use for invitations and greeting cards. At 152.4 × 241.3 mm (6 × 9½ in) it's long and slim rather than nearly square, which suits designs that read from top to bottom, such as wedding programs, dinner menus, slim invitations and gift certificates. It's usually made from heavier paper than an office envelope, often in color, and it opens along the long side.

Small businesses use it too, because it's as long as a standard business envelope but has far more room from top to bottom. That means a folded letter can travel together with a flat card, a voucher or a small brochure, which would never fit in a regular business envelope.

What fits, compared with #10 and A9

An A10 is cut so a card has a little room on every side, and our size chart gives its fit as "Cards up to 5¾ × 9 in". That margin matters when you order invitations, because some shops list their long cards as "6 × 9 in". Measure the card itself: one that really is six inches on its short side is as tall as the envelope and will only go in bent.

Folded paper fits as well, since a US Letter sheet folded in half or in three goes in with room to spare. The nearest sizes are easy to mix up, though. A #10 envelope is exactly as long as an A10 but much shallower, built for a Letter sheet folded in three and too narrow for a card. The A9 envelope is the next announcement size down, a little shorter and narrower, and made for half-letter cards, which fit an A10 too but slide around in it.

Printing the address at home

Most home printers can feed an A10, but only short edge first, because its long side is longer than the Letter or A4 width they take. The envelope icon on the tray or in the printer's manual shows which way the flap should face for your model. In the print dialog, pick the envelope size or a matching custom size and print at 100% ("actual size") rather than "fit to page", so the address lands where you placed it. Invitation envelopes are often thick, textured or lined, so try one spare envelope first and, if the printer struggles, put the address on a clear or matching label instead, as our guide on how to print envelopes explains.

The button on this page opens the label editor at the A10 size, with a return address at the top left, the recipient in the middle and the stamp corner left free. You can set each block in a script or serif font such as Great Vibes or EB Garamond, then print directly or save a PDF whose page is exactly the envelope's size. For a whole guest list, mail merge imports a spreadsheet and prints one envelope per address; mail merge needs a free account.

How an A10 is mailed in the US and the UK

USPS and Royal Mail both decide the format from an envelope's size, thickness and weight, and the A10 falls on different sides of the line in the two countries. The table shows each operator's limits next to what an A10 usually counts as.

In the US the A10 sits just inside the letter limits, so a flexible envelope that stays thin and light goes as a First-Class Mail letter. Any thicker or heavier, and it's charged as a large envelope, which costs noticeably more. With invitations, weight and embellishments are the things to watch: a heavy card with an RSVP card and a small reply envelope can come close to an ounce (28 g), and a vellum layer or a belly band can push it into the two-ounce price. Weigh one finished invitation, or try the stamp calculator, before you buy stamps for all of them.

Invitations often pay a surcharge on top of the weight price as well, because the sorting machines can't handle every envelope: USPS charges the non-machinable surcharge for envelopes that are rigid or lumpy or have clasps, string or buttons, and asks you not to put rigid objects such as coins, keys or flash drives in a letter. On a long envelope like the A10, keep the address parallel to the long side so it reads like a normal letter; our US stamp prices page shows what the surcharge costs. Embellished invitations are where the A10's size starts to cost you. For specially decorated envelopes, USPS lets you pay the non-machinable surcharge and ask the Post Office counter to hand-cancel them, and for wax seals, strings, ribbons and other decorations on the outside it advises not to send them exposed but to put the envelope inside another envelope. Because an A10 is already almost as tall as a letter can be, an outer envelope big enough to hold it goes over the letter limits in the table, so it's charged as a large envelope (flat), not a letter.

In the UK the A10 is a Large Letter even when it's thin and light, because its long side is just over a millimeter longer than Royal Mail allows for a Letter, at 1st and 2nd Class alike. A normal Letter stamp would leave it underpaid, so use a Large Letter stamp instead. If you'd rather pay the Letter price, a C5 envelope is a similar size that stays within the Letter limits and takes an A4 sheet folded once.