A9 envelope size and what fits in it

What fits in an A9 envelope

Cards up to 5½ × 8½ in fit inside, and that is exactly what you get when you fold an ordinary US Letter sheet in half. This is why the A9 is the usual envelope for cards printed at home: you design on Letter paper or cardstock, fold it once, and the card slides in with a little room on every side. The same half-Letter size is used for wedding and party invitations, event and church programs and folded menus, so one box of A9 envelopes covers most of what you are likely to send.

The fit matters in both directions. A card in the popular 5×7 format is too small for an A9: it slides around, its corners get bent in the mail and the envelope looks half empty, so an A7 envelope for 5×7 cards is the better choice. A card that is even slightly bigger than a folded Letter sheet won't close properly in an A9, and forcing it in creases the fold.

Why the A9's shape avoids the non-machinable surcharge

USPS sorts letters by machine, and a letter the machines can't handle pays a non-machinable surcharge on top of the normal postage. Shape is one reason: square envelopes, and envelopes that are very long and narrow for their height, don't run through the sorters and have to be handled by hand, which is why square envelopes cost more to mail even when they are light. The A9 is a comfortable rectangle, about one and a half times as long as it is tall, so its shape never triggers the surcharge.

What can still trigger it is what you add. A rigid card that won't bend, a button or clasp, or an address written vertically makes a letter non-machinable even in an A9. Decorations on the outside are a different matter. 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. If you want a wax seal or ribbon on your invitations, plan for an outer envelope and have one finished piece priced at the post office before you buy stamps for everyone.

How the A9 is mailed in the US and the UK

In the US an A9 counts as a letter, the cheapest format for anything in an envelope. USPS's letter limits are: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. An A9 stays inside that length and height with room to spare, so only thickness and weight need watching. A single half-fold card usually stays within the first weight step, which one Forever stamp covers, but a full invitation suite with a reply card and reply envelope can go into the next step. If it gets thicker than the letter limit, it is priced as a large envelope (a “flat”), which costs noticeably more, so weigh one finished envelope and let the stamp calculator work out the stamps before you buy them for everyone.

In the UK, where cards usually go in ISO envelopes such as C6 and C5, the A9 is less common, but if you post one it counts as a Letter. Royal Mail's limits for a Letter are “Up to 24 × 16.5 cm, 5 mm thick, 100 g”, and an A9 is smaller than that on both sides. It only becomes a Large Letter, at a higher price, when it is thicker or heavier than those limits, which is rare for one card but possible with a chunky embellishment or a stack of inserts.

Printing the address on an A9 at home

The button on this page opens the label editor at the A9 size, with a return address block top left, a recipient block in the middle and the stamp corner left free. You can move and restyle every block, then print directly or export a PDF the exact size of the envelope. For invitations, a script font such as Great Vibes looks good for the names, with a clear serif such as Cormorant Garamond for the street and city, because a very ornate address is harder for sorting machines to read.

Most home printers take envelopes short edge first through a rear tray or manual feed slot, and the A9's short edge is narrower than a Letter sheet, so width is rarely the problem. The flap and the thickness are, so check the icon on the tray to see which way the flap should face, set the scale to 100% instead of “fit to page”, and print a test on plain paper to hold against an envelope. For a guest list you can import addresses from a spreadsheet and print one envelope per guest; this mail merge needs a free account, which has a monthly limit (Pro is unlimited). If your printer won't feed envelopes at all, the guide to printing envelopes at home explains the switch to address labels.