A1 envelope size: the 4 Bar RSVP envelope

What fits in an A1 / 4 Bar envelope

The A1 is the smallest of the US announcement envelopes, a family that also includes the A2, A7 and A9. Stationery shops often list it as a 4 Bar, a trade name for the same envelope, so a box labeled either way holds the same cards. It is cut to carry RSVP and small cards (3½ × 4⅞ in), and a card of that size slides in with a little room on every side, so it doesn't catch on the flap. It also takes the common 3½ × 5 in response card, though with less room to spare in length, so that size is a snug fit.

In a wedding or party suite, the A1 is the reply envelope. It is addressed to the host and stamped in advance, the response card goes inside, and both travel to the guest tucked into the invitation's larger envelope, often an A7. Outside weddings, it holds gift enclosures and small thank-you notes that are handed over in person or packed in a parcel, where postal rules don't apply. If your cards are slightly too big, the A2 envelope is the next size up.

Mailing an A1: minimum size and postage format

USPS sets a minimum size for anything you mail, and an A1 is larger than that minimum by only a fraction of an inch each way. That makes it about the smallest envelope you can mail in the US, so don't trim it or swap it for a mini envelope. The small margin is enough to make it an ordinary First-Class Mail letter, a format USPS defines by its upper limits: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. A single reply card in an A1 is well under the first weight step, so one standard letter stamp covers it as long as the envelope stays flat and flexible. 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, so a very stiff layered card means extra postage. Leave a wax seal off the reply envelope as well. 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, and that is a step you can't ask a guest to take when they mail their reply. See US stamp prices for the current price.

Two situations call for something else. If you'd rather mail the response card as a postcard, its size has to fall within these limits: Rectangular card 5–6 in × 3½–4¼ in; bigger cards pay the letter price. The card the A1 is cut for is a fraction shorter than the smallest postcard, so it has to travel in its envelope. A 3½ × 5 in card just meets that minimum, though, so you can skip the reply envelope: address the card to yourself, add a postcard stamp, and your guest mails it back on its own. That only works on ordinary card stock, because USPS also limits how thick a postcard can be. A heavy letterpress, cotton or layered card is too thick to count as one and needs a letter stamp, plus the non-machinable surcharge if it is too stiff to bend, so take one to a post office counter and check it before you stamp the whole batch.

Guests abroad are the second situation. USPS sets a longer minimum length for letters going abroad, and an A1 falls short of it, so a thank-you note you would normally send in an A1 needs an A2 when it goes to someone overseas. The reply card doesn't work for guests abroad either: someone who mails it back from another country is sending an international letter under their own post office's rules, and US stamps are only valid on mail posted in the US, so a stamped A1 is no use to them. For guests abroad, leave the A1 out of the invitation and offer another way to answer, such as email or a website, or include an unstamped A2 that they can mail back with their own stamps if you want a paper reply.

In the UK, Royal Mail prices by format, and its smallest format is the Letter: Up to 24 × 16.5 cm, 5 mm thick, 100 g. The limits are the same for 1st and 2nd Class, so the choice between them is about speed, not size. An A1 is well within them, but they set the largest Letter, not the smallest, and we couldn't confirm that Royal Mail accepts an envelope this small. Check with Royal Mail or at a Post Office counter before posting a batch; for cards posted in the UK, a C6 envelope is the safer choice.

Printing the address on an A1 at home

An A1 is near the bottom of what home printers will feed. Every printer has a minimum paper size, and on some models this envelope is close to it or below it, so check your printer's manual or spec sheet for the smallest custom size before you buy a box of 100. If yours can't take it, or the envelopes go in crooked, print the address on address labels instead: one label from a standard 30-per-sheet US Letter sheet fits neatly on the front.

When the printer does take it, use the rear or manual feed tray if there is one and slide the guides in until they touch the envelope. Which edge leads and which way the flap faces differs between printers, so follow the envelope icon on the tray and print one test on plain paper cut to the same size first.

The button on this page opens the editor at exactly A1 / 4 Bar (3⅝ × 5⅛ in), with a sender block top left, a recipient block in the middle and the stamp corner left free. On a reply envelope the recipient is the host, so type your own address in the middle and delete the sender block or keep it for your return address. Every reply envelope carries the same address, so you don't need a mailing list: export one PDF and set the number of copies in your print dialog. A script font such as Great Vibes can echo the invitation, but on an envelope this small a clear font like Lora reads better for the street and city.