A4 format

ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today, although not in Canada, the United States, Mexico, or the Dominican Republic. 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 (except some envelopes) have the same aspect ratio, ${\displaystyle {\sqrt {2}}}$, within rounding to millimetres. This ratio has the unique property that when cut or folded in half width-wise, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next larger size in the same series.

In 1786, the German scientist Georg Christoph Lichtenberg described the advantages of basing a paper size on an aspect ratio of ${\displaystyle {\sqrt {2}}}$ in a letter to Johann Beckmann. The formats that became ISO paper sizes A2, A3, B3, B4, and B5 were developed in France. They were listed in a 1798 law on taxation of publications that was based in part on page sizes.

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