The international paper size standard is ISO 216. Paper size is based on the German DIN 476 standard for paper sizes.
ISO paper sizes are all based on a single aspect ratio of the square root of 2, or approximately 1:1.4142. There are different series, as well as several extensions.
The international paper sizes are included in Cascading Style Sheets (CSS): A3, A4, A5, B4, B5.