C5 envelope size specifications are 162 x 229 millimeters, or 6.4 x 9 inches, in accordance to standards set by the International Organization for Standardization. According to ISO 216 Standards, the C5 envelope is designed to be used for A4 size paper.
Size A4 paper can be folded in half to fit perfectly in a C5 envelope. The ISO was founded in 1947 and is responsible for developing the international specifications for products, services and best practices for almost all aspects of technology and business. ISO is an independent, nongovernmental organization with members representing 162 countries, excluding the United States and Canada.