9×12 envelope: size, what fits and how to mail it

What a 9×12 envelope is for

The 9×12 is the envelope to use when a document has to arrive flat, without a single crease. It is only a little larger than a sheet of US Letter paper, so contracts, résumés, certificates, school transcripts and signed forms go in exactly as they came out of the printer. You will find it in white and in brown kraft, with a peel-and-seal flap or a metal clasp.

Because nothing is folded, the pages are easier to scan, file or frame at the other end, and they look neater when they arrive. The trade-off is size: the envelope is bigger than a letter in both directions, which changes both what it costs to mail and how you get the address onto it.

What fits inside

A 9×12 takes US Letter (8½ × 11 in) and A4 paper unfolded, along with anything smaller: 8×10 photos, a half-letter brochure, a thin booklet, or a 6×9 envelope tucked inside as a reply envelope. A stack of 20 or so sheets of copy paper slides in easily. Much more than that and the envelope starts to bulge at the bottom, which can affect the postage, as the next section explains.

If your pages are folded once, a 6×9 booklet envelope is a snugger fit, and a few pages folded in thirds go in a #10 at the lower letter price, as long as the envelope stays thin, flexible and light enough to count as a letter. A thick folded wad is charged more. The one item people often expect to fit and doesn't is a two-pocket presentation folder. Those folders are made to the same 9×12 size as the envelope, so there is no room left around them, which means they need a 9½ × 12½ or a 10 × 13 envelope. The 10×13 also suits thicker bundles of paper.

Postage: a large envelope, not a letter

In the US, the Postal Service prices mail by shape first and weight second. A 9×12 is bigger than the largest size USPS accepts as a letter, so it can never go at the letter price. As long as it stays flexible, evenly thick and within the weight limit for large envelopes, it is charged as a large envelope, also called a flat. Large envelopes are priced by the ounce like letters, but they start from a much higher price for the first ounce, so a few pages cost noticeably more in a 9×12 than they would folded into a #10.

Here is a typical example. Five pages of ordinary 20 lb copy paper weigh about 23 g and a kraft 9×12 envelope about 15 to 20 g, so the whole thing comes to roughly 40 g, a little under 1½ oz. USPS rounds up to the next full ounce, so that envelope is priced as a 2-ounce large envelope. The stamp calculator turns the weight into a price and a number of stamps, though the post office scale has the final word.

A large envelope also has to stay flexible and evenly thick. If you add a stiff piece of cardboard, a pen or a bulky binder clip, it can be charged as a package, which costs more again. A very heavy or thick bundle also goes past the large-envelope limits and is charged as a package, and the post office counter decides that. In the UK the same envelope counts as a Royal Mail large letter rather than a letter, because it is wider and longer than the letter format allows. The closest European size is the C4, which is made for A4.

Printing the address

Most home printers take paper up to 8.5 in (216 mm) wide, and a 9×12 is wider than that whichever way you turn it. The easy answer is an address label. Print the recipient on a US Letter label sheet, such as the common 30-per-sheet Avery® 5160 size, stick it in the middle of the envelope and add a return address label top left. Labels are also the only sensible choice for clasp envelopes, because the metal fastener should never go through a printer.

If you own a wide-format printer that takes 11 × 17 in or A3 paper, you may be able to print straight onto the envelope, as long as its manual or rear slot handles envelopes this thick. The editor exports a PDF whose page is exactly the envelope size. Feed the envelope short edge first through the manual or rear slot, with the flap facing the way your printer's manual shows, and test on plain paper first. Printers differ in how well they handle an envelope this thick, and the guide on how to print envelopes covers feeding in more detail.

For a mailing list, such as an annual report sent to members, you can import the addresses from a spreadsheet and print one label per address. This is mail merge from Excel. It needs a free account, and free accounts get five batch exports a month.