ISO international standard paper size

　　ISO International Standard Paper Size: ISO’s paper system is based on the 2-dimensional root of the Gaucho ratio (1:1.4142), meaning that the area of a sheet of paper is half that of a sheet of paper one size larger than it is and twice that of a sheet of paper smaller than it is.

　　ISO 216 is the international standard for paper size defined by the International Organization for Standardization (ISO) and used by most countries in the world today. The well-known A4 paper size is defined by this standard.

　　ISO 216 defines three series of paper sizes, A, B and C. The C series of paper sizes are mainly used for envelopes.

　　The ISO 216 format follows a 1:√2 ratio; two sheets of paper placed together with the same width-to-height ratio and the same sides. This feature simplifies many things, such as: photocopying two sheets of A4 paper down to one sheet of A4 paper; photocopying one sheet of A4 paper up to one sheet of A3 paper; photocopying and enlarging half of A4 paper to one sheet of A4 paper, etc., will use up all the space in the paper, no more, no less.

　　A-Series

　　A0 is defined as a paper with an area of one square meter and a width to height ratio of 1 : √2. The next paper sizes A1, A2, A3 ……, etc., have the long edge as the wide edge of the previous number one paper and round to the largest millimeter value possible for the area closest to, but not exceeding, the standard value. Note that the wide edge is not simply equal to half the long edge of the previous sheet, or the long edge of the current sheet divided by √2.

　　B-Series

　　The B-series paper size is the geometric average of the A-series papers with the same number as the previous number. For example, B1 is the geometric average of A1 and A0. Similarly, the C-series paper size is the geometric average of the A and B-series papers with the same number.

　　C Series

　　C-series paper sizes are mainly used for envelopes. An A4-size sheet of paper can just fit into a C4-size envelope; fold the A4 paper into A5 paper, it can just fit into a C5-size envelope, and so on.