There are a number of paper sizing standards. The three series shown in the table below are the most commonplace for publishing and printing and are: ‘A’ sizes, ‘B’ sizes and ‘SRA’ sizes.
The international paper size standard, ISO 216, defines the ‘A’ sizes and 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 one m². Rounded to millimetres the A0 paper size is 841 by 1,189 millimetres (33.1 × 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 × 297 mm). The ‘A’ sizes are therefore the everyday size of finished printed items.
Printing requires that each edge of the finished sheet is trimmed during the manufacturing process and therefore the printed area is slightly larger than the finished page size (‘bleed’ is added to each page). This requires a larger printing sheet to be put through the printing press and is the basis behind the ‘B’ series and ‘SRA’ series.

