C6 envelope size and what fits inside

What a C6 envelope is for

The C6 size comes from the ISO paper system, in which each C envelope is made to hold the A sheet with the same number flat. At 114 × 162 mm (4½ × 6⅜ in), a C6 takes an A6 card (105 × 148 mm, about postcard size) with a few millimetres to spare all round. And because A4 folded in half twice is exactly A6, it also takes an ordinary letter folded in four, so it works for short letters as well as cards.

In the UK and much of Europe, C6 is the standard greeting-card envelope: many birthday, thank-you and Christmas cards sold in the UK are A6 and come with C6 envelopes, which makes C6 one of the most common greeting-card envelopes there. That also makes it the size to buy for home-printed cards and A6 invitations. It works for folded letters too, although a DL envelope, which takes A4 folded in three, looks more like business post.

What fits inside a C6

Anything A6 or slightly smaller fits: a flat or folded A6 card, a sheet of A5 folded once, or A4 folded in four. US Letter folded in four (about 108 × 140 mm) fits as well, and so does a standard 4×6 in photo, which makes a C6 handy for sending prints to family. A folded card in its envelope weighs around 15 g, about the same as a letter of two A4 sheets, which matters when you think about postage.

The limit is the envelope itself, less a few millimetres so the contents can slide in, which means something slightly bigger than A6 still fits but anything much bigger won't go in. A 5×7 in card is too long and needs an A7 envelope, while an A5 card you don't want to fold belongs in a C5 envelope. Thickness is the other limit, because each A4 sheet folded in four becomes four layers, so beyond two or three sheets the envelope starts to bulge and won't seal flat.

Which postage format a C6 counts as

Royal Mail prices post by format first and weight second, and a C6 is well inside the size limits of a Letter, the cheapest format. At around 15 g, a card or short letter is also far below the Letter weight limit, so it goes at the Letter rate. What can push it into the dearer Large Letter format is thickness. A Letter has to stay within these limits: Up to 24 × 16.5 cm, 5 mm thick, 100 g. An ordinary card in a C6 is well within them and needs one 1st or 2nd Class stamp, but a card with a badge or a pop-up insert can be thicker, and it is then charged as a Large Letter despite its small size.

In the US, a C6 falls within the USPS letter-size range and is rectangular rather than square, so a normal card or folded letter in it is mailed as a letter. USPS sets the letter limits like this: Up to 11½ × 6⅛ in, ¼ in thick, 3.5 oz. A card thicker than that is no longer a letter, and USPS treats it as a large envelope, which costs more and must be flexible and evenly thick, so a rigid or lumpy card of that thickness should be priced at the counter. Below that thickness, an envelope that is stiff, lumpy or uneven still can't go through the sorting machines and costs extra. In either country, if a card feels thick or heavy, have it checked at the counter before you stamp it, because the post office's own scale and template decide.

Printing the address at home

A C6 measures 114 × 162 mm (4½ × 6⅜ in), so even its long edge is well under the 216 mm width most home printers take, though the icon on your tray decides which way round it goes. The editor's PDF is exactly that size, so pick C6 or a matching custom size in the print dialogue and print at 100%. The guide to printing envelopes at home covers feeding and test prints.

The red, green or kraft envelopes in A6 Christmas card packs hide black ink, so print white labels from an A4 sheet of 21 instead and stick one in the middle of the C6, clear of the stamp corner. For 40 cards, import the addresses from a spreadsheet with a free account and print one label or envelope per name.