C Paper Sizes

The C series formats are defined in ISO standard 269, originally published in April 1985. This ISO standard was withdrawn in 2009 without a replacement, however C series sizes are still specified in several national standards today. The C series is primarily used for envelope sizes. The area of a C series sheet is the geometric mean between the sizes of the A and B series sheets of the same number. For instance, C4 is slightly larger than A4 yet slightly smaller than B4. Consequently, an A4 sheet fits neatly into a C4 envelope, and an A4 sheet plus C4 envelope together fit into a B4 envelope. Some national standards also define envelope formats combining adjacent C sizes to achieve an approximate 2:1 aspect ratio, such as DIN C6/C5 (114mm x 229mm). This C65 envelope size accommodates an A4 sheet folded into thirds.