The dimensions of A3 paper are 11.7 inches by 16.5 inches, or 297 millimeters by 420 millimeters. The dimensions of A4 paper are 8.3 inches by 11.7 inches, or 210 millimeters by 297 millimeters. The dimensions of A5 paper are 5.8 inches by 8.3 inches, or 148 millimeters by 210 millimeters. The dimensions of these paper sizes and the rest of the A series are defined by the international ISO 216 standard.
The A series paper sizes are used by most of the world with the exception of North America. The United States and Canada are the only two countries that have officially refrained from adopting the A series dimensions, and an additional handful of countries, including Mexico, Columbia, Venezuela, Chile and the Philippines, have officially accepted it but continue to use the U.S. Letter format.
The international standard is based on the Deutsches Institut für Normung 476 standard. Before Word War II, it was adopted by nine European countries. After the war, an additional 36 countries adopted the standard, and by 1975, the International Organization for Standardization decided to accept it as an ISO standard. DIN 476/ISO 216 is also the standard for the United Nations.
Similar to the B and C series, the A series comprises 11 sizes from A0 to A10. Various individual countries also have their own variations to these paper sizes, including Germany, Sweden and Japan.