A4 paper is the standard paper size used worldwide, with the exception of countries in North America. The US, Canada, and Mexico have standardized 8.5 inch by 11 inch (about 21.5 cm by 27.9 cm) paper, typically referred to as "letter" size. A4 paper is slightly longer, at about 8.27 inches by 11.75 inches (21 cm by 29.7 cm). This and other types of "A" paper sizes are based on a metric measurement system and established through international European standards.
Basis for Different Sizes
A4 paper and similar sizes are based on metric measurements. An A0 piece of paper, when spread flat, has an area of one square meter and is the largest size at 84.1 cm by 118.9 cm (about 33 inches by 46.75 inches). A0 paper, cut in half, becomes size A1 at about 59.4 cm by 84.1 cm (or 23.38 inches by 33 inches). A1 cut in half is A2, and so on through A6.
This produces paper with different sizes, but all with a 1:1.414 height-to-width ratio. European standards also cover B paper sizes. B0 paper is a meter wide and 1.414 meters tall (about 39.37 inches by 55.75 inches); B1 is B0 cut in half, and so forth through B5.
The value of this system for paper sizes is that each size is proportionately equal to the next. A4 paper, for example, is about 21 cm by 29.7 cm (about 8.27 inches by 11.75 inches), while A3 is 29.7 cm by 42 cm (around 11.75 inches by 16.5 inches). If someone has text or images on a sheet of A4 paper and wants to enlarge it onto A3 paper, it can be done quite easily without losing any of the content of the document. This proportion, which uses the square root of two, or 1.414, is known as the Lichtenberg Ratio, named for the German physicist who first noted its usefulness.
Size and Standardization
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. While the standards have been updated occasionally, the sizes used have been maintained since the early 20th Century.
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 cover stock is 250 g/m2.
North American paper continues to be determined by Imperial Measurement. Standard letter size paper in the US is 8.5 inches by 11 inches (about 21.5 cm by 27.9 cm), while legal size paper is 8.5 inches by 14 inches (around 21.5 cm by 35.56 cm). This means that someone trying to enlarge a document from letter to legal loses information because the North American format does not use a constant ratio between sizes. Engineers and other people working on international projects often use A4 paper and similar European standards to avoid confusion and make conversions simpler and more precise.