Paper Size - The International Standard: ISO 216 - A Series

A Series

The international paper size standard, ISO 216, is based on the German DIN 476 standard for paper sizes. ISO paper sizes are all based on a single aspect ratio of square root of 2, or approximately 1:1.4142. The base A0 size of paper is defined to have an area of 1 m2. With this definition and the given aspect ratio of square root of two, one can calculate the sides of an A0 sheet as follows: The long side is 1 metre multiplied by the square root of the square root (that is, the fourth root) of 2 and the short side is 1 metre divided by the same. Rounded to millimetres, the A0 paper size is 841 by 1,189 millimetres (33.1 in × 46.8 in).

Successive paper sizes in the series A1, A2, A3, and so forth, are defined by halving the preceding paper size along the larger dimension. The most frequently used paper size is A4 210 by 297 millimetres (8.3 in × 11.7 in).

The significant advantage of this system is its scaling: if a sheet with an aspect ratio of is divided into two equal halves parallel to its shortest sides, then the halves will again have an aspect ratio of . Folded brochures of any size can be made by using sheets of the next larger size, e.g. A4 sheets are folded to make A5 brochures. The system allows scaling without compromising the aspect ratio from one size to another—as provided by office photocopiers, e.g. enlarging A4 to A3 or reducing A3 to A4. Similarly, two sheets of A4 can be scaled down and fit exactly 1 sheet without any cutoff or margins. Weights are easy to calculate as well: a standard A4 sheet made from 80 gram/m2 paper weighs 5 grams (as it is one 16th of an A0 page, measuring 1 m2), allowing one to easily compute the weight—and associated postage rate—by counting the number of sheets used.

The advantages of basing a paper size upon an aspect ratio of were already noted in 1786 by the German scientist and philosopher Georg Christoph Lichtenberg. Early in the 20th century, Dr Walter Porstmann turned Lichtenberg's idea into a proper system of different paper sizes. Porstmann's system was introduced as a DIN standard (DIN 476) in Germany in 1922, replacing a vast variety of other paper formats. Even today the paper sizes are called "DIN A4" (pronounced: "deen-ah-fear") in everyday use in Germany and Austria. The term Lichtenberg ratio has recently been proposed for this paper aspect ratio.

The main disadvantage of the system is type does not scale the same way; therefore, when a page is resized, the type set on it loses legibility as the proportion between the type's x-height, page margins, and leading are distorted. When trim is involved, as in the manufacture of books, ISO 216 sizes are generally too tall and narrow for book production (see: Canons of page construction). The distortion is even more pronounced with printed sheet music. European book publishers typically use metricated traditional page sizes for book production.

The DIN 476 standard spread quickly to other countries. Before the outbreak of World War II, it had been adopted by the following countries:

During World War II, the standard was adopted by Uruguay (1942), Argentina (1943) and Brazil (1943); and afterwards spread to other countries:

By 1975 so many countries were using the German system that it was established as an ISO standard, as well as the official United Nations document format. By 1977 A4 was the standard letter format in 88 of 148 countries. Today the standard has been adopted by all countries in the world except the United States and Canada. In Mexico, Costa Rica, Colombia, Venezuela, Chile and the Philippines the US letter format is still in common use, despite their official adoption of the ISO standard.