C4

The C4 paper size has dimensions of 229 x 324 mm, making it the fifth largest size in the C series of envelopes defined in ISO 269 (withdrawn in 2009). With a surface area of 0.07 m2, C4 envelopes are a common oversized format. As part of the C series based on geometric means between A and B series sheets, C4 falls between A4 (210 x 297 mm) and B4 (250 x 353 mm). One key use of C4 envelopes is that they can fit a standard A4 sheet of paper, making C4 a popular envelope size for mailing documents, brochures, and letters. The C4 and other C sizes stem from early 20th century German efforts to define paper sizes based on geometric principles, later included in ISO standards.