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. The North American paper standards were far more informally adopted in the late 19th century. Exact origins of the letter-sized paper are argued by historians, but may be based on the size and weight of foolscap paper, which was commonly used at the time. Foolscap typically measured 16.75 inches by 13.5 inches (about 42.5 cm by 34 cm). Some argue the North American standard letter size came from cutting the foolscap in half.

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.

North American paper continues to be determined by Imperial Measurement. This means paper is weighted for specific uses, such as printer paper or cover stock. Each paper type is given a basis size in inches, which are then multiplied by 500 to produce a bond weight—the weight of 500 sheets of that particular type of paper.

The weight of European paper is based on the metric system and is standardized under ISO Standard 536, originally developed in 1976 and updated in 1995. European and other non-American papers are weighted based on the number of grams per square meter (G/M²) for each type of paper. A4 paper is available in most standardized paper weights; A4 printer paper's weight is 90 G/M², while A4 cover stock is 250 G/M².