C5

The C5 paper size has dimensions of 162 x 229 mm, making it the sixth largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.035 m2, C5 is a common envelope size. As part of the C series based on geometric means between A and B series sheets, C5 falls between A5 (148 x 210 mm) and B5 (176 x 250 mm). A key use of C5 envelopes is mailing documents, letters, and other content printed on A5 paper, as C5 envelopes can neatly fit a folded A5 sheet. The C5 and other C sizes have their origins in early 20th century German efforts to define paper sizes based on geometric principles, later incorporated into ISO standards.