C8

The C8 paper size has dimensions of 57 x 81 mm, making it the ninth largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.004 m2, C8 is a less commonly used envelope size. As part of the C series based on geometric means between A and B series sheets, C8 falls between A8 (52 x 74 mm) and B8 (62 x 88 mm). C8 envelopes can be used for mailing small items such as gift cards, keycards, and slim paper documents. However, the similar but slightly larger C7 size is more popular. The C8 and other C sizes originate from early 20th century German efforts to define paper sizes based on geometric principles, later included in ISO standards.