A4 paper is the standard paper size used worldwide, with the exception of countries in North America. The United States (U.S.), Canada, and Mexico have standardized on 8.5 inch by 11 inch (about 21.5 cm by 28 cm) paper. A4 paper is slightly longer, at 8.3 inches by 11.7 inches (about 21 cm by 29.7 cm).

Paper sizes were standardized in Germany in 1922, and adopted throughout Europe in 1930, via Standard 216 of the International Standards Organization (ISO). The ISO is a non-governmental entity that publishes international standards. Standard 216 was most recently updated in 2007, adding standards to define which direction of the paper is parallel to the machine, which can affect paper curl.  

A4 and the other A paper sizes are based on metric measurements. An A0 piece of paper, when spread flat, equals one square meter. A0 paper, folded in half, is a size A1. A1 folded in half is A2, and so on through A6. This produces paper with different sizes, but all with a 1.414:1 height-width ratio. European standards also cover B paper; B0 paper is a meter wide and 1.414 meters tall; B1 is B0 folded in half, and so forth through B5.