The #9 envelope: the return envelope that fits inside a #10

Why a #9 fits inside a #10

The #9 is a little narrower and a little shorter than the standard #10 business envelope, and that small difference is the reason the size exists. A business that mails a bill, a statement or an invoice in a #10 can tuck a #9 inside, already addressed to itself. The customer then only has to put the payment in it and send it back. Because the #9 lies flat inside the larger envelope, nobody has to fold it, and it comes out ready to use.

You have probably received one already. It's the plain envelope that falls out of a utility bill or a credit card statement, with the company's payment address printed in the middle. Freelancers and small businesses that still send paper invoices use the same approach. Schools, clubs and event organizers use it for RSVP-by-mail forms that are too big for a small reply card. If you need the outer envelope too, the #10 envelope page gives its exact size and what fits in it.

What fits in a #9 envelope

The #9 is designed around a US Letter sheet (8½ × 11 in) folded in three, the same fold you'd use for a #10. Folded that way, the sheet is 8½ in wide, so it fits along the length of the envelope with a little room to spare, and the flap still closes easily. That makes the #9 the right size for a one-page reply form, a signed agreement coming back to you, or a survey filled in by hand.

Payments are the other classic content. A personal or business check lies flat in a #9 without folding. So does the remittance slip that many invoices carry at the bottom, which is the tear-off strip with the account number and the amount due. The customer tears off the slip, adds a check and seals the envelope, so you get both back together. Two or three folded sheets still fit, but every extra sheet adds weight, and past a certain weight the envelope needs more postage, as the last section explains.

Printing your address on a #9

On a reply envelope, your own address goes in the recipient position in the middle, because that is where the envelope is headed. The top-left corner and the stamp corner are left for the customer. The button on this page opens the editor at the #9 size with a sender block top left and a recipient block in the middle, and the stamp corner left free. Type your business address into the recipient block and delete the sender block so the corner stays blank. The guide to addressing an envelope shows how to set out each line.

Most home printers take paper and envelopes up to 8.5 in wide. The long side of a #9 is longer than that, so it has to go in short edge first. The small envelope symbol on the tray or rear slot shows which side faces up and where the flap should be, though printers differ. The editor exports a PDF whose page is exactly the size of the envelope. Print one, check where the address lands, and then print as many as you need, since every reply envelope carries the same address. If your printer won't take envelopes at all, print your address on a sheet of address labels instead and stick one on each envelope.

Postage in the US and the UK

In the US, a #9 counts as an ordinary First-Class Mail letter. The USPS letter limits are "Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz", and the size of a #9 sits well inside them. A reply holding one folded sheet and a check weighs well under an ounce, so one regular letter stamp covers it. What usually makes a #9 cost more is weight, not thickness, because the regular stamp only covers the first ounce. USPS says 1 oz is about 4 sheets of regular 8½ × 11 in paper in a rectangular envelope, so if your reply holds more than that, weigh it before you stamp it. A heavier reply still counts as a letter up to the weight in the limits above, but it needs extra postage for each further ounce. If it goes over either the thickness or the weight in those limits, USPS charges it as a large envelope (flat), which costs more again. Current prices for each ounce and each format are on the US stamp prices page.

In the UK, Royal Mail treats a #9 as a Letter, the cheapest format, in both 1st and 2nd Class. Its Letter limits are "Up to 24 × 16.5 cm, 5 mm thick, 100 g", and a #9 meets them comfortably as long as it stays thin. If you pack it thicker than that, it becomes a Large Letter and needs a Large Letter stamp. Keep in mind that the #9 is an American size. It is slightly longer than a DL, the usual British business envelope, so it won't fit inside one. The UK stamp prices page lists what each format costs.