C5 envelope size and what fits in it

What a C5 envelope is for

The ISO envelope sizes are built around the A paper sizes: each C envelope is made to hold the A sheet with the same number flat, with a little room around it. A C5 is therefore the envelope for A5, the half-A4 sheet (148 × 210 mm) used for flyers, booklets, notepads and greeting cards. That is also the answer if you searched for an "A5 envelope size", because the ISO system has no envelope called A5, and shops selling "A5 envelopes" are almost always selling C5.

The size earns its place for a second reason. Folding A4 once gives you exactly A5, so a C5 lets you send a full page with a single crease, which is why people use it for CVs, contracts, certificates and reports that would look tired after being folded in three for a DL envelope. Don't confuse it with US names like A2 or A7, which are American card envelope sizes and unrelated to A paper.

What fits inside a C5

An A5 sheet slides in flat with a few millimetres to spare all round, and A4 folded once fits in exactly the same way. In practice a standard C5 takes several sheets of folded A4, an A5 brochure, or an A5 greeting card, which is usually made from A4 card folded in half. US Letter paper folded once goes in too, although it sits closer to the ends because Letter is slightly wider than A4.

Smaller items fit but slide about, so an A6 card or a postcard is better off in a C6 envelope, the size made for A6 and for A4 folded in four. The real limit of a C5 is thickness rather than area. A plain C5 has no gusset (an expanding fold along the sides), so a thick bundle makes it bulge and the flap struggles to seal, and the extra thickness can also raise the postage.

Letter or Large Letter: postage for a C5

In the UK, a thin C5 carrying a few sheets of paper can often go at Royal Mail's Letter price. What usually turns it into a Large Letter is thickness or weight: a card with a ribbon, a booklet or a thicker bundle of paper soon goes past the Letter limits, and it then usually becomes a Large Letter at the higher price, which is why many people treat every C5 as a Large Letter to be safe. Large Letter has its own thickness and weight limits as well, though, so a padded C5 holding a small book, or a bundle stuffed until the flap barely closes, can go past them and is then charged as a Small Parcel. If you are unsure, the size gauge at the Post Office counter settles it.

In the US the picture is different, because a C5 is slightly taller than USPS allows for a letter. Even a thin C5 is therefore treated as a large envelope, which USPS also calls a flat, and costs more to post. If you are sending folded Letter paper within the US, a 6×9 envelope does the same job and still counts as a letter as long as it stays thin and light.

Printing the address on a C5

Most home printers can take a C5, but only short edge first, because its long side is wider than the widest paper they accept while its short side is narrower than A4. Use the rear or manual feed tray if your printer has one, since that path bends the envelope less. Which way up and which way round the envelope goes depends on the printer, so mark the top of a plain sheet, print a test on it and copy what you see. In the print settings, choose C5 or a custom size that matches the envelope, and switch off any "fit to page" option so the address isn't shrunk.

The button on this page opens the editor with a C5 template already laid out: your return address top left, the recipient in the middle and the stamp corner left clear. Change or move any block, add a logo if you like, then print or export a PDF the exact size of the envelope. Padded or gusseted C5s often jam in home printers, and for those address labels are the easier route. If you are writing to a whole list, you can import a spreadsheet and print one envelope or label per address with a free account.