ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today, with the United States as an important exception. The standard defines the "A" and "B" series of paper sizes, including A4, the most commonly available size. Two supplementary standards, ISO 217 and ISO 269, define related paper sizes; the ISO 269 "C" series is commonly listed alongside the A and B sizes.

All ISO 216, ISO 217 and ISO 269 paper sizes have the same aspect ratio, Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle 1:\sqrt{2} . This ratio has the unique property that when cut or folded in half lengthwise, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next size up.

The advantages of basing a paper size upon an aspect ratio of √2 were already noted in 1786 by the German scientist Georg Christoph Lichtenberg, in a letter to Johann Beckmann. The formats that became A2, A3, B3, B4 and B5 were developed in France, and published in 1798 during the French Revolution, but were subsequently forgotten.^{[citation needed]}