Within the A Size of international paper sizes, which paper size measures 297 mm x 420 mm (11.7 x 16.5 in)?

There are 11 sizes in the A series of paper sizes, which are designated the letter and numbers A0 to A10. When dividing the long side of the paper by the short side, the total equals the square root of 2 (√2), which is approximately 1.41421. The measurements of A3 paper are 297 mm x 420 mm (11.7 x 16.5 in).

The German scientist Georg Christoph Lichtenberg was the first to notice the benefits of using paper with an aspect ratio of √2. This is because the ratio does not change when the paper is folded in half. The result of folding an A3 sheet of paper is an A4 sheet (210 x 297 mm or 8.25 x 11.75 in). Similarly, folding an A2 sheet (420 x 594 mm or 16.2 x 23.375) in half results in A3.

Introduced in 1922 as DIN standard, the A Series of paper was first used in Germany, where the paper sizes are still referred to as "DIN A3", "DIN A4" and so forth. DIN stands for 'Deutsches Institut für Normung', which translates as the 'German Institute for Standardisation'.

By the outbreak of World War II in 1939, several European countries had adopted the A Series, including Belgium, Finland, Hungary, Italy, the Netherlands, Norway, the Soviet Union, Sweden and Switzerland. The rest of Europe followed suit in the decades after the war. By 1975, so many countries were using the German system that the United Nations established it as the international standard. As of 2022, only the United States and Canada do not use the A Series of paper size.