A6 envelope size: the 4×6 envelope for photos and cards

Why the US A6 envelope isn't ISO A6

In the United States, A6 is one of the announcement envelopes, the family of sizes made for invitations, greeting cards and photos. It measures 120.7 × 165.1 mm (4¾ × 6½ in), so a standard 4×6 print slides in with a little space on every side, which is why people call it the 4×6 envelope. The flap usually runs along the long side, and you address it in landscape, with the long edge at the bottom.

The confusion comes from the ISO paper series used in the UK, Europe and most other countries, where A6 is a sheet of 105 × 148 mm, a quarter of an A4 page. The US envelope only borrowed the name, and the ISO envelope actually made for A6 paper is the C6 envelope. In practice the mix-up only hurts in one direction: an ISO A6 card drops easily into a US A6 envelope, but a card cut to fit the US envelope is a fraction too wide for a C6, so it won't go in.

What fits inside an A6 envelope

An A6 envelope is a little larger than a 4×6 print, which gives it room for a few other common sizes as well. Check what you're planning to send against this list:

• Cards and photos up to 4½ × 6¼ in (4×6), the job the envelope is designed for.
• A 4×6 index card, recipe card or postcard you'd rather send in an envelope.
• An ISO A6 card or flyer (105 × 148 mm), with room to spare.
• A sheet of A4 or US Letter paper folded in half and then in half again.

A folded sheet of paper fits but moves around inside, so it works best as a short note tucked in with a photo or card. If you make your own folded cards, keep the finished card within the size above and allow a couple of millimeters for thick stock, because the fold adds bulk at the edge. A 5×7 invitation or photo is too big for an A6, and for that size you need the A7 envelope.

Printing the address at home

Most home printers can print on an A6 envelope, since it is narrower than Letter paper and only slightly bigger than the 4×6 photo paper many inkjets already take. It usually feeds short edge first, in portrait, with the flap closed, but which way the flap should face depends on the printer. The quickest check is to mark one corner of a plain sheet with a pencil, print a test page and see where the mark ends up. In the print dialog, pick a paper size that matches the envelope and print at actual size rather than "fit to page", so the address lands exactly where you placed it.

The editor opens at exactly this envelope size and exports a PDF the same size, so nothing gets scaled on the way to the printer. Print the envelopes before you put the cards in, because flat, empty envelopes feed far more reliably, and use the rear or manual feed slot if your printer has one; on a laser printer, let the envelopes cool before you stack them, as the heat can soften the glue on the flap. If your printer won't take small envelopes, or you have a whole stack of cards to send, printing address labels on a sheet is often quicker, and the guide on how to print envelopes covers printing a single envelope or a whole list in more detail.

Postage in the US and UK

For USPS, an A6 envelope with a card inside counts as a letter, because its size and proportions fall within what the letter-sorting machines handle, which is not true of square greeting-card envelopes. A card and envelope together usually weigh 10 to 20 grams, roughly half an ounce, so it normally goes at the basic letter price. What can change that is the contents rather than the size: a wax seal, a ribbon, a button or a card too stiff to bend makes the envelope nonmachinable, and those letters cost more because they have to be sorted by hand. Weight and thickness matter too: a card with several photos tucked in can go over the weight a single letter stamp covers, and an envelope thicker than a letter is allowed to be is charged as a large envelope (what USPS also calls a flat), not as a letter.

Royal Mail treats a thin A6 envelope as a Letter, its smallest format, so a single card goes on an ordinary 1st or 2nd class stamp. Royal Mail also measures thickness, though, which means a card with a badge, layered decorations or several photos inside can count as a Large Letter and cost more. In both countries, write or print the address along the long side, and if a batch of cards has something heavy or bulky in it, have one weighed and measured at the post office before you stamp them all.