10×13 envelope: size, what fits and how to address it

What a 10×13 envelope is for

The 10×13 is one of the largest everyday catalog envelopes, and it's the one for contents that have to arrive flat and won't fit anything smaller. A sheet of US Letter or A4 paper goes in unfolded with room on every side, which leaves space for a pocket folder, a thick report, a booklet or a catalog. Law offices, schools and real estate agents keep a box of them for closing documents, application packets and manuscripts.

Because nothing is folded, the papers arrive ready to sign, scan or file. The cost of that convenience is that the post office never treats a 10×13 as a letter, and the envelope is too wide for a normal printer. Both points decide how you address it and what it costs to send.

10×13 or 9×12: when the extra inch matters

For a few loose sheets of Letter paper, the smaller 9×12 envelope is usually enough, since the paper fits with about half an inch to spare across the width. That margin is also its weak point. A stack of paper has thickness and the envelope has to wrap around it, so every bit of thickness uses up room the sheets need, and the 10×13 adds an inch in both directions to absorb it.

That inch matters most in three situations. A standard two-pocket presentation folder is already about as big as a 9×12 envelope, so it won't go into one at all. A thick report with a cover and dividers bulges in a 9×12 but closes easily in a 10×13. And when you need documents sent back flat, such as a contract to sign, you can tuck an addressed and stamped 9×12 return envelope inside a 10×13. If none of these applies, the lighter 9×12 envelope is the better choice.

Printing the address: why a label is easier

Most home inkjet and laser printers are built around Letter paper, so their trays stop at the width of a Letter sheet. A 10×13 envelope is wider than that even on its short side, which means it won't feed in either direction, and pushing it in usually ends in a crumpled envelope or a paper jam. That's why the buttons on this page open a US Letter sheet of 30 address labels (the Avery® 5160 size) instead of the envelope. You print the sheet on your ordinary printer and stick a label in the middle of the front, with your return address on a second label in the top left corner and the top right corner left free for stamps.

Labels are also quicker when the same packet goes to many people. If the addresses are in a spreadsheet, the editor can import the list and print one label per address, which needs a free account; printing address labels from Excel shows how. Many wide-format printers for 11×17 or A3 paper can print on the envelope itself, as the questions below explain.

Postage: a large envelope in the US, a parcel in the UK

In the US a 10×13 can't go as a letter, because USPS letters must stay within these limits: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. It goes as a large envelope instead, which USPS also calls a flat, and the limits for those are: Bigger than a letter, up to 15 × 12 in, ¾ in thick, 13 oz; must be flexible and evenly thick. A 10×13 fits that comfortably as long as it stays bendable and evenly thick. Add something stiff or lumpy, such as a binder clip or a USB stick, or go over the weight limit, and USPS prices it as a package.

The UK is stricter about width. Royal Mail's Large Letter has these limits: Up to 35.3 × 25 cm, 2.5 cm thick, 750 g. This envelope measures 254 × 330.2 mm (10 × 13 in), so its short side is a few millimeters wider than a Large Letter may be, and Royal Mail would charge it as a small parcel. For unfolded A4 documents in the UK, a C4 envelope is the size that stays within the Large Letter format.

To work out the stamps, weigh the envelope with everything inside. A Letter sheet of ordinary copy paper weighs about 4.5 g, so twenty sheets in a card folder, plus the envelope, come to roughly 6 oz depending on the folder. Enter your own weight in the stamp calculator to see how many stamps that means, and keep in mind that the post office's scale has the final say.