ISO 216

ISO 216 specifies international standard paper sizes, used in most countries in the world today. It is the standard which defines the well known A4 paper size. The standard was originally adopted by DIN in Germany in 1922, although some of the formats contained therein were independently invented in France during its revolution and later forgotten.

ISO 216 defines three series of paper sizes: A, B and C. Series C is primarily used for envelopes.

Paper in the A series format has a 1:sqrt(2) aspect ratio, although this is rounded to the nearest milimetre. A0 is defined so that it has an area of 1 square metre, prior to the above mentioned rounding. Successive paper sizes in the series A1, A2, A3, etc., are defined by halving the preceeding paper size parrallel to its shorter side, again prior to rounding. The most frequently used paper size is A4, which is 210 x 297 mm.

The B series formats are geometric means between the A series format with the same number and the A series format with one higher number. For example, B1 is a geometric mean between A1 and A0. Likewise, the C series formats are geometric means betweent the B series format with the same number and the B series format with one higher number, e.g. C2 is the geometric mean between B2 and B1. (There is also an incompatible Japanese B series, defined on the basis of the arithmetic mean instead of the geometric mean.)

Prior to the adoption of ISO 216 worldwide, many different paper formats were used internationally. These formats did not fit into a coherent system and were defined in terms of non-metric units. The ISO 216 formats are organized around the ratio 1:sqrt(2), thereby greatly simplifying magnification during copying; magnifying during copying with other paper formats is more difficult. The main holdbacks in this area are the United States of America and Canada, which continue to use the Letter, Legal, and Executive system (Canada uses a P-series of sizes, which are nothing more than the US paper sizes rounded to metric dimensions).