The standardized values for paper sizes known today as DIN formats were first defined by the German Institute for Standardization (DIN) on August 18, 1922, in the DIN standard DIN 476. They classify the ratio between the width and height of a sheet of paper, which is the same for all sheet sizes, namely 1: √2.
Only with this ratio does the next smaller sheet created by folding centrally over the long side remain geometrically similar to the original sheet.
This aspect ratio, on which the DIN standards are based, goes back historically to the 18th century and was already specified by Georg Christoph Lichtenberg in 1786. Forgotten for a long time, it was taken up again in 1910 by Wilhelm Ostwald and subsequently disseminated by Walter Porstmann and enforced as a DIN standard in 1922.
With its specifications on the A and B series, the German standard served as the basis for the European and international equivalent EN ISO 216, which in turn has been adapted in almost all countries. The only differences are usually in the permitted tolerances. As a purely national standard, DIN 476-2:2008-02 Paper end sizes – C series is still valid.
DIN ISO 216
Title: Writing paper and special groups of printing material – end formats – A‐ and B‐series and marking of machine running direction
Short description: ISO paper formats
Last edition: Dezember 2007
DIN ISO 216
Area: Paper and paper products for data processing, offices and schools
Title: Paper-end formats – C‐series
Short description: none
Last edition: February 2008
ISO and DIN paper sizes
There are four series (A and B according to ISO and DIN, C and the original D according to DIN), each of which is divided into eleven classes numbered in descending order of size from 0 to 10.
The nominal area of an A0 sheet is one square meter, but due to the rounding of the side lengths to whole millimeters, the real areas in the A series deviate from one square meter or whole fractions thereof. The same applies to whole multiples of √2 for the other series. Because of the permitted length tolerances, the real areas can deviate even further.