Envelope sizes: US, ISO and Japanese chart

How envelope sizes are named

US envelopes come in two series, each with its own naming. Commercial envelopes, the long ones used for bills and business letters, carry a number: the higher the number, the bigger the envelope, and the #10 (4⅛ × 9½ in) is the one almost everyone uses. Announcement envelopes for cards and invitations carry an A and a number, such as the A2 (4⅜ × 5¾ in) or the A7 (5¼ × 7¼ in). That A has nothing to do with A paper, so an A7 envelope holds a 5 × 7 in card while a sheet of A7 paper is smaller than a postcard. Larger US envelopes, the booklet and catalog sizes, are simply named after their size in inches.

Most other countries use ISO sizes, which are built around A paper. A C envelope holds the A sheet with the same number flat, so a C4 takes A4 and a C5 takes A5, and each also takes the next sheet up folded once. B envelopes are a little larger again for bulky contents, and DL is made for A4 folded in three. Japan has its own JIS sizes in three families: 長形 (naga-gata) are long envelopes for folded letters, 角形 (kaku-gata) are wide envelopes for flat documents, and 洋形 (yō-gata) are Western-style envelopes for cards and invitations.

Which envelope fits what you're sending

The quickest way to choose is to start from what goes inside, because the way you fold it decides the size. The table pairs the most common contents with the usual envelope in each system, and the chart above gives the exact size of each one.

The answer to "what size envelope for A4?" therefore depends on the fold: a DL for three folds, a C5 for one and a C4 for none. A 5 × 7 in wedding invitation goes in an A7, and a card made from a Letter sheet cut in half and folded goes in an A2. The A7 measures 133.4 × 184.2 mm (5¼ × 7¼ in), so a 5 × 7 in card has about ⅛ in to spare on each side, which is just enough room for it to slide in without catching.

The standard envelope in each country

When people ask for a "standard" or "letter" envelope, they mean the one their country's paper fits after it's folded in three, because that's still how most letters are sent. In the US that's the #10, also called the business envelope, at 104.8 × 241.3 mm (4⅛ × 9½ in). In the UK and the rest of Europe it's the DL at 110 × 220 mm (4⅜ × 8⅝ in), and in Japan it's the 長3 at 120 × 235 mm (4¾ × 9¼ in); both are made for A4. These are the sizes sorting machines handle best and every office supply store keeps in stock.

Sticking to these sizes also keeps postage down, because postal operators price letters by size and thickness as well as weight. An envelope bigger than a standard letter, such as a 9 × 12, a C4 or a 角2, pays a higher large-envelope rate in all three countries, and more still once it is thicker or heavier than the large-letter limits allow. A square, stiff or lumpy one can cost more even when it is small, because the letter machines can't sort it. A stiff insert can be enough to cause that, because 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. On the same principle, USPS also charges more for envelopes that are square or vertical (taller than they are wide). If you're choosing invitation envelopes, check what square envelopes cost to mail before you order them.

Printing the address on large envelopes

Most home printers take paper up to 216 mm (8.5 in) wide, the width of a US Letter sheet. Letter and card envelopes are narrow enough for most of them, but the large flat ones aren't: a C4 is 229 × 324 mm (9 × 12¾ in), a catalog 9 × 12 is 228.6 × 304.8 mm (9 × 12 in) and a 角2 is 240 × 332 mm (9½ × 13⅛ in), so even their short side is too wide. The home printer column in the chart shows which sizes fit most home printers, so check your own printer's envelope settings before you print a batch.

For those three, print the address on an address label and stick it on, which also works on padded or very thick envelopes. Labels and printed envelopes save time on a long mailing list too: if your addresses are in a spreadsheet, a free account lets you print one envelope or one label per row, as how to print envelopes explains step by step. For a single envelope that your printer can take, the print link on its row in the chart opens that size with the return address top left, the recipient in the middle and the stamp corner left free.