#10 envelope size and what fits inside
A #10 envelope measures 104.8 × 241.3 mm (4⅛ × 9½ in). It's the standard US business envelope, made for a sheet of Letter paper folded in three, and it's the one most bills, invoices and business letters arrive in.
- Size
- 104.8 × 241.3 mm (4⅛ × 9½ in)
- What fits
- Standard business envelope: Letter paper folded in three
- Printing
- Fits most home printers; the icon on the paper tray shows which way round to feed it.
- Also called
- #10, no. 10, com10, com-10
Why the #10 is the standard business envelope
The #10 is built around US Letter paper. Fold a Letter sheet into three equal panels, the way nearly every business letter is folded, and the result slides into a #10 with enough room to go in easily but not enough to shift around. Because American offices print almost everything on Letter paper, this became the default envelope for letters, invoices, statements and checks, and printer drivers list it by name, usually as "Envelope #10" or "Com-10".
Window envelopes depend on that fold. A #10 window envelope has a clear panel on the front, and business letter templates put the recipient's address exactly where that panel lands once the page is folded in three, so the letter addresses its own envelope. The #9 is the other half of the system: it's slightly smaller in both directions, so it fits inside a #10 next to the letter. That's how utility companies and charities send you a reply envelope, and it's the easy way to do the same if you want customers to mail back a check.
What fits inside a #10 envelope
Apart from a few sheets of Letter paper folded in three, a #10 takes the long, narrow things made for business mail: tri-fold brochures, rack cards, checks, gift certificates and slim flyers. A sheet of A4 folded in three fits as well, with a little less room top to bottom because A4 is taller than Letter. In the UK and Europe, though, the usual envelope for that fold is the DL envelope, which is shorter than a #10.
Square and squat items are a poor fit. Greeting cards and invitations are cut for card envelopes such as the A2 or A7, and folding one to fit a #10 spoils it. A thick stack of folded sheets is the other problem, because the envelope bulges, seals badly and may stop counting as a letter. For longer documents, a 6×9 envelope takes Letter folded once, and a 9×12 takes it unfolded.
Printing the address on a #10 at home
Most home printers take paper up to the width of a Letter sheet, and a #10 is longer than that, so it feeds short edge first. Choose "Envelope #10" as the paper size in the print dialog, then check the small envelope icon by the tray or feed slot to see which way the flap should face, since this differs between printers. Test with a spare envelope first, with a pencil mark in one corner so you can see which way it went through. On a laser printer the heat can wrinkle envelopes and warp the film on window envelopes, so buy envelopes sold as laser-safe. The guide to printing envelopes at home has more on feeding and on printing a whole list.
The button on this page opens a #10 layout with the return address block in the top left corner, the recipient block in the middle and the stamp corner left free. Type both addresses, pick a font, then print directly or export a PDF whose page is exactly the size of a #10. For a mailing list, a free account lets you import a spreadsheet and print one envelope per address, with five batch exports a month on the free plan. If your printer won't take envelopes at all, print address labels on Letter sheets instead, 30 to a page in the Avery® 5160 size, and stick them on.
Postage for a #10 in the US and UK
In the US, a #10 is a letter by USPS rules. Its size sits well inside the letter limits, so a #10 with a few folded sheets goes First-Class Mail with ordinary stamps, priced by weight. Overfilling it changes that: an envelope that gets too thick or too heavy is priced as a large envelope (USPS calls these flats), and one that is rigid, lumpy or closed with a clasp pays a nonmachinable surcharge. A #10 sent abroad goes as an international letter at USPS's own rates.
The UK is less obvious. Royal Mail sorts mail into Letter, Large Letter and parcel sizes by length, width, thickness and weight, and a #10 is longer than the envelopes British letters usually travel in. Royal Mail's limits for a Letter, 1st or 2nd Class, are these: Up to 24 × 16.5 cm, 5 mm thick, 100 g. A #10 measures 104.8 × 241.3 mm (4⅛ × 9½ in), which puts its length just over that maximum, so in the UK a #10 always counts as a Large Letter, even with a single sheet inside, and a Letter stamp on its own would leave it underpaid. If you post from the UK regularly, use DL envelopes instead, because they're the standard UK letter envelope and fit within the Letter size.
Print the addresses instead of writing them
The free label editor opens at the size of your envelope (#10 (4⅛ × 9½ in)) with a sender and a recipient block. Type one address and print it, or import your address list with a free account and print one envelope per address. It runs in your browser: nothing to install.
Print on this envelope →FAQ
Is a #10 envelope the same as a DL envelope?
No, though they do the same job on different sides of the Atlantic. The DL is the international size for A4 folded in three, while the #10 is made for US Letter, so a DL is a little shorter and a little taller. A4 folded in three fits in either one, but Letter folded in three is a tight squeeze in a DL because Letter is wider than A4.
Where do the stamp and return address go on a #10 envelope?
The stamp goes in the top right corner, your return address in the top left, and the recipient's address roughly in the middle. Keep the strip along the bottom edge clear, because USPS sorting machines print a barcode there.
How many sheets of paper fit in a #10 envelope?
Up to about five sheets of ordinary copy paper fold together and still close neatly. Beyond that the folded stack gets thick and the envelope bulges, so a 6×9 or 9×12 envelope is the better choice for longer documents. Postage is a separate question: each sheet adds weight, and four or five sheets can already push a letter past the first weight step, especially on heavier letterhead, so weigh it before you choose stamps.
Does a window envelope cost more to mail?
No. A window doesn't change the price: a #10 window envelope pays the same as a plain #10 of the same weight, which is a letter in the US and a Large Letter in the UK. What matters is that the whole address shows through the window, with a little space around it, even when the letter slides around inside. If part of the address hides behind the edge of the window, sorting machines may not read it and the letter can be delayed.
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Updated 7 November 2026