The international paper size standard ISO 216 was adopted in Europe in the 19th century and is now used in almost all countries. It 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 1:1.41421.
The advantage of basing a paper size on an aspect ratio of the square root of 2 is that when a sheet is folded, the length to width ratio does not change.
The A series is the most widely used standard for general printing and letterhead stationery. The dimensions of the A series paper sizes are defined by the ISO 216.
The most common paper size is A4 (210 x 297mm) and the largest is A0, with an area of 1 square metre.
So how does it all work?
Using A4 as a starting point, it’s easy to calculate the other sheet sizes. To go up the scale, simply multiply the shorter dimension by two. And to get to smaller sizes, just divide the longer dimension by two.